Diegetic Theme Cameo

The usage of a show's theme music within an episode. Generally only used if the theme is itself a freestanding song rather than a Title Theme Tune.

The musical version of an Opening Shout-Out. Flirts with Left the Background Music On. See also Leitmotif.

Not the same thing as Title Theme Drop, which is when a title theme is also used in the context of the work itself such as if a remix of the title theme plays when fighting the final boss of a video game, though there may be some overlap at times, if both are used within the same work. Basically, if a character hums the theme tune, a band plays it or it's on a ringtone or something -- if it's diegetic-- then it's this trope. If the theme song plays as the soundtrack over the episode, then it's a Title Theme Drop. If the lyrics of the theme tune (or any other song) are used in dialogue, it's Waxing Lyrical.

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 * Many products back in the day had songs about their products, such as the songs for KitKat, Almond Joy/Mounds. Nowadays, since singing a song about your product seems old-fashioned, some of these products that had songs will now just play a short instrumental version of these old songs somewhere in the commercial. One problem: if you're not old enough to remember these songs when they had words, you'll have no idea that these songs are meaningful as a Theme Tune Cameo and aren't just random songs.

Anime & Manga

 * In episode 7 of the anime Hana Saku Iroha, Ohana hums the opening theme in the bath.
 * In the original Japanese version of the first season of Sailor Moon, its theme song, "Moonlight Densetsu", is apparently a current hit. It is sung several times by different characters, including once as a duet in a talent show. (It should probably be noted that the "fighting evil by moonlight" lyrics were created for the North American dub; the original is actually about finding one's "miracle romance".) Interestingly, a slow version of the same tune is played by the star-shaped locket in the first season -- this locket is also present in a flashback to the Moon Kingdom, which existed in the distant past. Perhaps the song is older than it seems...
 * Although it's not the show's theme tune, the anime Full Metal Panic! regularly uses a variation of the theme song from The A-Team as background music. In the follow-up series Fumoffu, the song is also used as the main character's cellphone ringtone, and one other character remarks how it sounds "just like the theme song of one of those old American TV series".
 * In the 2005 series of the Anime Ah My Goddess, Belldandy and Holy Bell sing a haunting rendition of the theme song, Open Your Mind, as part of a magic spell. In the second season she sings it again during a talent contest with Sayoko, who sings the first season ending theme.
 * Similarly, during the Festival Episode of Love Hina, Seta asks Haruka (voiced by Megumi Hayashibara, who recorded the theme) to sing for him "just like she used to." She then begins singing a slow, almost melancholy, version of the very hyper theme song. Most of the other Hinata girls scattered throughout the festival each join in, until there is a chorus of voices singing on the final line.
 * Characters in Martian Successor Nadesico frequently sing snatches of the opening theme, most often just the title line "You Get to Burning" -- and usually not very well. At one point Erina even chastises Yurika for messing up the lyrics.
 * And in the "Idol Singer" episode, Yurika sings an expanded version of the ending theme.
 * In the Christmas episode, Seiya makes up Christmas-related lyrics to sing to the tune of the opening theme.
 * Magical Girl Lyrical Nanoha uses the theme song as a cellphone ringtone.
 * Azumanga Daioh does the same thing with a character's Image Song.
 * Tomo has her Image Song as a cellphone ringtone.
 * Osaka and Sakaki sang their image songs at the karaoke. Yomi does the same and epically fails. As if Rie Tanaka couldn't sing...
 * More on Sakaki, her Image Song appears again in the radio show that both Tomo and Yomi were listening to, only it's listed as being sung by Yuu Asakawa, who is actually Sakaki's seiyuu.
 * Tokyo Mew Mew also has one of its theme songs as a cell phone ring, and one episode involves two characters bonding over the song and making plans to go to the artist's concert.
 * Hazumi and Hatsumi listen to the Anime Theme Song to Yami to Boushi To Hon no Tabibito on a train in episode 7.
 * Abenobashi Mahou Shoutengai: In a world that may or may not be the original one from which they come, Sasshi and Arumi stand in the shopping arcade proper and look around while the closing theme of the show plays, muzak-style, over the PA system.
 * In Keroro Gunsou, while doing chores around the house, Sgt. Keroro sometimes sings his own version of the show's closing theme.
 * Another episode had Sumomo singing a few bars of the show's theme.
 * Another one had the first ending as Fuyuki's ringtone.
 * And another one had the first season's summer-only Dance Festival ending used for a dance rehearsal, about two years after the fact.
 * Even the manga gets into the act; the first chapter of volume 19 features Keroro singing one of the closing themes from the anime at a karaoke club.
 * In an episode of Princess Princess, the "princesses" and student council go for karaoke to train for the school festival. The Student Council President begins to sing the opening Anime Theme Song in one room, whereupon the "princesses" retreat to a different room by themselves... and sing the ending theme. Then, when the actual concert comes, they sing the theme to Cutey Honey.
 * In Nana, the Black Stones' first hit song is the OP for the series, and a hit single from their chief rival band is the series ED. Both songs show up several times in the show itself.
 * In Ouran High School Host Club, Kaoru and Hikaru have the series ED as one of the ring tones on their cell phones.
 * In an episode of Rockman EXE (Mega Man NT Warrior outside Japan), the protagonist's mother is humming the opening notes (from when they start singing) of the theme song.
 * The sequel series, Ryuusei no Rockman, reuses the opening as the first song a popstar composes on her own after angsting about only singing what's given to her. In the first episode she sings it in, she uses it to distract The Dragon Gemini Spark from a battle. Yes, he/they (don't ask) stand there and stare at her for the entire ninety seconds the song goes on. So does Rockman, actually, not attacking as per the diversion she intended the song for until the last couple seconds of it.
 * Also done in Ryuusei no RockMan Tribe. The song Misora sings at the concert in the last episode is the theme song, "Kizuna Wave." The animators even used footage from the opening (although that may have because the ending was rushed to completion).
 * In one episode of Rockman EXE Axess Netto hums the ending theme of that series while he's in the shower.
 * The opening of the anime series Gankutsuou (We Were Lovers by Jean-Jacques Burnel) appears in-show as the song one of the characters composes and plays on the piano in several occasions.
 * Re:Cutie Honey plays with this by having a villain sing the theme song... but replacing the lyrics about how beautiful Cutie Honey is with how much she hates Cutie Honey. She even gets the Mooks into the action.
 * The Pretty Cure franchise does this way too many times:
 * In one of the later episodes of Futari wa Pretty Cure, the choir club has to find a song to sing... and they pick the show's ending theme.
 * And they even dubbed the song in the English Dub of Pretty Cure instead of replacing it, although the ending theme was an instrumental of "Together We Are Pretty Cure", the dub's opening theme.
 * The same theme (Get You! Love Love?) would occasionally be hummed by a recurring character, even once after the show became Futari Wa Pretty Cure Max Heart and the ending theme was replaced with a new one, named MuriMuri? Ariai! INJanai!?
 * In Yes! Pretty Cure 5 GoGo, the theme song plays over the attack sequence.
 * Fresh Pretty Cure's first ending is apparently part of Trinity's song list (played in the first episode). The second ending is played during a dance contest in the final episode.
 * The orchestral version of You Make Me Happy! played during the episode of the next season when.
 * Heartcatch Pretty Cure gets into the act in episode 41, using both the opening and second ending themes for the respective parts of a puppet show.
 * In the Pretty Cure All Stars movies, when the respective Pretty Cure teams come on during the climax of the movie, their theme songs played.
 * In Smile Pretty Cure, a instrumental, slow version of Yay! Yay! Yay! is background music.
 * Elfen Lied's theme, "Lilium," was frequently used within the show -- Kouta had a music box which played the tune, and was playing it when he and Lucy met as children. In the present, the theme was used to allude to their former relationship, the memories of which Kouta had suppressed after Lucy killed his family. For instance, it made Nyuu very upset, even though she didn't know why, and even caused her to revert to Lucy.
 * Lucia's music box in Mermaid Melody Pichi Pichi Pitch plays the show's first ending theme, and sometimes the episode titles and important lines are actually song titles (usually "Yume no sono saki e"). Being a series that incorporates music all through, there are too many instances of characters singing their own songs to list.
 * Death Note has Nightmare's "Alumina", the first ending tune, as the ringtone on Matsuda's mobile phone, and the first opening tune on Misa's.
 * Also "Alumina" can be heard on Misa's laptop in episode 13. Still it goes largely unnoticed by most fans.
 * Episode ten of Paranoia Agent, in which a frantic victim accidentally turns on his car radio and the show's opening theme song plays.
 * The Lupin III movie Crisis in Tokyo features Lupin whistling the franchise's theme song.
 * There's also Fujiko humming it while driving a truck in the series proper.
 * In the TV special "Stolin Lupin", Lupin's ringtone played the theme song.
 * One dub episode of Yu-Gi-Oh GX has Jaden singing part of the theme song.
 * Another dub episode has Fubuki/Atticus singing (badly) a Parody version of the theme song. "Rock on! Get your rock on! Everybody rock now!"
 * In the original version of the original series, it happens too, with Anzu's ringtone being the second season opening song.
 * The Yu-Gi-Oh 5 Ds dub does this twice. At one point, the hyperactive Cloudcuckoolander scientist is caught singing the theme. Yusei also had it playing on a music player in one episode, while fixing his Duel Runner.
 * The ending theme of Rurouni Kenshin, sung by the voice actor of Sanosuke, was discontinued; in the next episode, Sanosuke is walking down a street and singing it.
 * In the Dn A OVA, Karin Aoi and Junta's mother sing karaoke, performing the series's closing and opening themes respectively.
 * In episode 9/14 ("Someday in the Rain"; see Anachronic Order) of The Melancholy Of Haruhi Suzumiya, Haruhi sings a snippet of the show's ending theme while undressing Mikuru. Don't ask.
 * And in another example, Kyon's little sister hums a bit of the ending theme while stealing Kyon's scissors.
 * In Haruhi-chan episode 4, Mikuru hums some of the aforementioned ending theme while getting her maid clothes.
 * Lucky Star's Konata uses the ending theme from another Kyoto Animation show, The Melancholy of Haruhi Suzumiya, as her ringtone. The song also shows up in a few other places, including an arcade rhythm game.
 * She also hums the opening theme for The Melancholy of Haruhi Suzumiya while playing her Brand X MMORPG, replacing certain lines with MMORPG-specific phrases.
 * One episode has a scene where Konata and her friends attend a concert featuring an Image Song from The Melancholy of Haruhi Suzumiya.
 * This is a real world concert Suzumiya Haruhi no Gekisō that the characters 'attended'. The 'person' singing on stage is Aya Hirano, credited as such (she is also the VA of Konata and Haruhi) and is a fairly good likeness of the actress.
 * The ending themes for Lucky Star up to episode twelve are often themes to other (non-Kyoto Animation) series, anime or otherwise. The others are general popular tunes.
 * And yes, Lucky Star's own theme makes an appearance, hummed by one of the girls in episode 6.
 * And in episode 24, when the Dancing Theme is animated in full. (Well, full shortened version, anyway.) In the same episode, Minoru is interrupted by his ringtone, which is his own version of "Koi no Mikuru Densetsu"(one of the endings).
 * Mitsuki's idol songs from Full Moon O Sagashite are all used as ending themes throughout the series.
 * Hayate the Combat Butler not only has the characters doing karaoke with their Image Songs and the show's Theme Tune in one episode, the ending segment of the show is a clear advertisement for said songs. With No Fourth Wall, this is commented on.
 * The Arcus Prima in Simoun has a phonograph-like machine which is often heard playing background music from elsewhere in the series. It almost escapes this trope, but it finally plays one of the themes (the end theme) in episode 25.
 * In one episode of Ai Yori Aoshi, Kaoru of all people sings karaoke to the end theme. A different song (sung by Tina) plays over that episode's credits, perhaps indicating some kind of theme conservation law. Aoi is also humming the opening theme at the beginning of episode 15.
 * The vocal version of the aptly named instrumental track "Nagisa" from the Clannad visual novel, "Dango Daikazoku", is used as the first ending for the anime series, and is sung in the second episode by Nagisa herself. It is also said to be Nagisa's favorite song. Also, as evidence of how much his girlfriend is rubbing off on him, Tomoya whistles it in the third episode of the After Story. It makes several other appearances throughout the anime, too.
 * In The Movie, Nagisa sings the song and it is also used as background music.
 * Used often in Neon Genesis Evangelion with "Fly Me to the Moon".
 * The anime also contains several musics based on the opening theme, "Cruel Angel's Thesis." Also note that in an interesting reversal, the ending theme of the movie Death and Rebirth ("Thanatos -- If I Can't Be Yours") was based on a music from the original anime.
 * The Hot Springs Episode in Maburaho has Kuriko listening to the theme song on her portable music device on the train.
 * Alicia from Aria at one time hums the opening tune of the series while lighting some candles in her home.
 * Inverted in the second Digimon Tamers movie. Parasimon forces Ruki's memories of her father to the surface, and of a song she used to sing for him--which just so happens to the movie's ending song. While Ruki's seiyuu recorded the full song (which was later released on CD), the version over the end credits was performed by Ai Maeda, the regular singer for nearly all of the franchise's ending songs. Later, during her birthday party, Xiaochun and Ruki's mother sing karaoke of the series' opening and first ending, "The Biggest Dreamer" and "My Tomorrow".
 * While we're at Tamers, an interesting one: Ritsuko's ringtone is Ravel's Boléro. While technically not originally a Digimon song, the track was part of Digimon Adventure's soundtrack, usually played within the context of the real world.
 * In Digimon Adventure, the song Mimi sang to wake TonosamaGekomon was the first ending, I Wish. Mimi was also voiced by Ai Maeda, so they just had to use the prerecorded audio until he woke up.
 * In Digimon Adventure 02, there are two scenes of Yamato's band playing his image songs, "Walk On The Edge" and "The Key to Granting Wishes."
 * Also in Adventure 02, Hikari hums "Break Up", the Armor Evolution theme.
 * In Digimon Xros Wars the Young Hunters Leaping Through Time, Nene performs two songs at her concert: "New World", the second opening theme to the preceding Digimon Xros Wars in which she was a main character, and on two occasions performs snippets of "Stand Up", the current opening theme. Being that the songs' artists, Twill, don't do voice-acting, both songs were covered by Houko Kuwashima for their use here, just not particularly well...
 * Super Robot Wars Alpha 3 has Macross 7's Basara write a song that he wanted to record with Lynn Minmei; it doesn't actually get played until the final battle, where it inspires the heroes to say "Screw Destiny" and fight the insanely powerful Final Boss. The song is, of course, the theme song for Alpha 3 itself. Bonus points since Yoshiki Fukuyama of JAM Project (the band who originally did the song) provides Basara's singing voice.
 * Hideaki Asaba from Kare Kano is seen (and heard) listening to the series' opening theme, Tenshi no yubikiri, in one of the later episodes of the anime.
 * An early episode of Ultra Maniac involves the characters going to a karaoke bar and singing snippets of the opening and ending themes as duets. Singable lyrics were written for the dub. Nina, a transfer from the Magic Kingdom, sings a song from Doraemon, since it's the only one she knows.
 * In one episode of The Vision of Escaflowne, a character finds and plays a music box. The song it plays is "Yakusoku wa Iranai", the show's opening theme.
 * Konjiki no Gash Bell starts playing the first theme almost any time the main characters reveal a new Finishing Move. And dozens of other times as well.
 * The first episode with Apollo shows him playing the opening theme for children on a flute-like instrument.
 * If you listen closely in episode 27 of Get Backers, "High School Girl vs. Recovery Service," you can hear some random background girls singing the recently replaced first opening.
 * Miya does this in Koi Koi 7, but her particular rendition of the show's OP is...terrible.
 * One office worker in Dai-Guard can be heard humming the show's opening tune while playing with an action figure of the eponymous robot.
 * In one episode Akagi sings part of the Dai-Guard Take Off music when he's teasing Ibuki, but it's only in the dub
 * Ibuki has the main theme in chiptune form as a ringtone.
 * Maruko can sometimes be heard humming Odoru Poroponkin (depending on which season you're watching is either the show's opening or ending theme) in Chibi Maruko Chan.
 * The Junior Detective League in Detective Conan keeps singing openings and endings themes in the series.
 * The second season of Hidamari Sketch ends with the cast singing the first season opening.
 * In Macross Frontier, nearly all OpeningThemes and EndingThemes (save the first OP) are sung by the in-universe IdolSingers. This crosses over into Opening Shout Out territory  when sequences from both opening credits are used with parts of the second OP playing in the background.
 * Although not quite as blatant, Macross Plus evokes this trope with "Voices," the first and last song heard in the OVA, sung by Myung at various points, and used in instrumental form as her Leitmotif.
 * In an episode of Ranma One Half, some characters perform part of the first season opening song for karaoke.
 * One of the opening songs also appears in a TV show that Ranma is watching.
 * It goes further than that: in the Christmas Episode OAV, the characters that would compose the DoCo supergroup (Akane, female Ranma, Kasumi, Nabiki, and Shampoo) perform the "Equal Romance" ending theme for the benefit of their party guests. Additionally, an entire two-part OAV, Nettou Uta Gassen, consists of the entire cast performing a multitude of songs in-character.
 * After a particular denial of a normal body for Ranma, he closes the episode in girl form, sadly singing the refrain to that season's end theme: "Don't cry, China Boy..."
 * Similarly, in Mezzo DSA, Mikura brings her karaoke machine on a car ride (to Harada's annoyance) and sings the theme song. This would be fine if she didn't sing it all the way through... more than once. You'll never see a more obvious "rake scene" in anime.
 * In Gravion Zwei, during a Hot Springs Episode the maids are without their cool control room during an attack, so Klein Sandman gives them karaoke microphones so they can they sing the Super Robot Combining Mecha Transformation Sequence theme song.
 * The bar in Tokyo where Lucy meets Akagi in FLAG has a lounge singer singing the show's ED song, Lights. The singer herself looks suspiciously like Eri Nobuchika, the real-life singer of the ending theme, and the bar itself is named Lights.
 * Top wo Nerae!2 Diebuster episode 4 has a moment where Nono hums the beginning of "Active Heart," the opening of Gun Buster.
 * Monster uses its theme song as background music at restaurants, dance clubs, and such. The sneaky part is that it's played cabaret-style -- so slow and laid-back that you can easily fail to recognize it.
 * Hell Girl has used at least two of its themes as ringtones. Also noteworthy: SNoW, who sings the OP of seasons one and two, has a cameo on an advertisement at the end of Futakomori's theme.
 * In the first series of Dragonball (at least in the French dub, which had a different opening) Goku hums the Japanese theme song before kicking ass.
 * Tayutama's ninth episode has a brief Karaoke Box session that has characters singing the anime's opening and ending, as well as the opening of the fandisc of the Visual Novel it was based on.
 * Cromartie High School plays with this in Episode 3: Hmm hmm hm-hm-hmm hmm-hmm hm-hm-hm-hmmm-hm-hm' You'd think it was related to the show, but it's actually Subliminal Advertising.
 * A later episode had the ending credits tune played on a cellphone. That was Mechazawa's little brother. Maybe it is a leitmotif.
 * Played with in an early episode of Pokémon, "Here Comes the Squirtle Squad". Ash is humming along to the BGM, which is not the opening theme to the show, but rather the title screen music from the video game. Lampshaded by the narrator, who brings it to the audience's attention.
 * The Japanese version of the previously-banned-then-not-anymore-and-then-again-yes episode, the Beach Episode, has Koujiro (James) exclaiming that he and Musashi (Jessie) will "go through the fire and through the water". The sentence is part of the original opening's lyrics.
 * Any time Brock has to sing, he will always sing "Takeshi's Paradise" (Takeshi being Brock's name in Japan), one of the end themes in the Johto arc; apparently it's the only song he knows.
 * And other OP's and EP's have been used as background music too, but most of the time this is lost in the dubs.
 * The first scene of episode 69 of Shugo Chara has Kusukusu humming the show's opening theme.
 * In Duel Masters, after Knight is accosted by a mysterious cloaked servant of the Temple, he leaves wondering what the other person could possibly be thinking. Her thoughts are revealed afterward. "Who's the kid with the spiky hair? Shobu!"
 * The Hot Springs Episode of The Galaxy Railways sees a group of Bridge Bunnies singing the show's opening theme at a talent competition. They've dragged the main character into singing with them. In drag. His love interest, understandably, responds by downing a glass of sake.
 * In an episode of Suzuka Hatari sings The shows opening theme at a karaoke parlor.
 * In episode 14 of Sasami Magical Girls Club, Itoki sings both the opening theme and the first closing theme at a karaoke parlor. The scene begins with her singing the opening theme, then later in the same scene she starts singing the original ending theme and the screen behind her even shows a glimpse of the original ending credits.
 * Taken to ridiculous levels in Hime-chan No Ribbon. Apparently, "Egao no Genki" (the series' opening) and the band that performs it, SMAP, is extremely popular in Hime-chan's world -- in one episode, Hime-chan pretends to have invited SMAP to a school event to save the drama club's reputation by turning into the band members and walking out on stage one at a time, there was almost a scandal when Hibino was caught hugging Hime-chan disguised as one of the band members, and the song pops up all over the place.
 * If you carefully listen in Episode 31 of the Kirby anime, Mike Kirby will be singing to the ending theme of Japanese version, "Kihon wa Maru". Kirby sings it in "Kirbyese" though (his "Poyo" sounds), though if you listen carefully you can hear him say "Kirby" and "Dedede" at the appropriate points in the song.
 * In another episode of the anime, Escargon sings a song about how much he loves (in a platonic way, not like that) King Dedede, set to the tune of the first opening, Kirby March.
 * During battles with some "demon beasts", one of the songs that plays contains a remix of part of the Japanese theme song (which made it into the game Kirby Air Ride as the Kirby Melee music).
 * Outlaw Star's opening theme is played by a gigantic advertising spaceship in the episode "Final Countdown." The first ending theme also appears twice, sung by Melfina.
 * In You're Under Arrest, the episode in which Sena is introduced has the first opening theme, 100 MPH no Yuuki, play on the radio.
 * Natsumi also sings part of the second opening theme in one episode.
 * An episode of Black Jack (I'm not sure which one.) had a woman singing the second closing theme song on stage.
 * In a episode of Buso Renkin, Hideyuki Okakura hums the theme song.
 * Axis Powers Hetalia is guilty, too. In one episode when all the countries are in line at the supermarket doing something rude, obnoxious, or disruptive....and South Italy cuts in line while whistling the main theme.
 * In episode 47, Little Italy interrupts Austria's piano recital while singing Maru Kaite Chikyuu. In the middle of it, Italy's voice changes, and Austria not realizes that Italy hit puberty, he found out that Italy wasn't female.
 * At the end of World Series episode 34, Italy hums Hatefutte Parade while spying until Germany tells him to stop.
 * In Peach Girl, Momo and Sae's ring tone is an instrumental version of the opening theme.
 * In (one) BLAM Episode of Ergo Proxy, Vincent is trapped on a game show, and one question is to name the composer of a song. Then the theme tune plays, and we find out that Vincent guessed correctly. The Fourth Wall was never the same again.
 * The protagonist of Kamikaze Kaitou Jeanne hums her opening on one occasion.
 * In the first Sorcerer Hunters OAV, Carrot asks "What's up, guys?"...a reference to the opening theme song of the Sorcerer Hunters TV series.
 * In the second season of Strike Witches, Shirley hums the first season's ED.
 * In one of the early episodes of the dub of the Witchblade anime, a character hums part of the chorus of XTC, the show's first opening.
 * In Touhou: A Summer Day's Dream, Suika hums a snippet of the opening theme as she helps Reimu clean up the shrine.
 * In episode 8 of Slayers Evolution-R, a scene opens up with Gourry humming along to the soundtrack, only to be scolded by Xelgadis.
 * At one point, Ryo Saeba whistles the beginning bars of Ai yo Kienaide, only to be cut off when confronted with a beautiful lady.
 * Excel Saga takes this to the next level, with the song being performed by animated representations of the voice actresses who normally sing it, themselves being dressed in the distinctive outfits of the main characters and being derided as knockoffs thereof.
 * The English dub of Burst Angel has the ending theme recited as a poem in one episode.
 * In one episode of Windy Tales, Nao sings the ending theme song as part of an audition.
 * When the cast of Yumekui Merry goes to a karaoke bar, we hear someone down the hall performing the show's theme song, and doing a remarkable job of sounding like the original singer.
 * Madoka Magica: Madoka listens to the OP in a record store during episode one. And, in episode 6, a techno version is played on Kyouko's game of Dog Drug Reinforcement.
 * Nerima Daikon Brothers combines this with Waxing Lyrical in one episode: while apologizing to Ichiro, Hideki and Mako quote lines from the opening theme.
 * The 2007 Tamagotchi movie uses the ending theme as Tanpopo's ringtone. This is done twice: at the beginning of the movie, and.
 * A fairly subtle one in Cardcaptor Sakura has a random woman humming the second season opening as she walks to the mailbox.
 * Guilty Crown has Inori singing the first ending theme in-series  She also sings "Euterpe," the song that the series began with (but isn't the official opening theme), to herself when she's lonely... and also sings it at the Cultural Festival the high school puts together.
 * Rin in Kodomo No Jikan hums the opening theme, "Rettsu! Ohime-sama Dakko" - basically, "Let's! Carry me like a princess" - to herself in the bath. An earlier scene plays on the theme's title, where Rin asks her teacher to carry her like a princess.

Comics

 * In a comic featuring Venom as the main character, he's seen swinging building to building singing a song, with lyrics featuring himself, obviously meant to line up with the classic Spider-Man theme song.
 * In one of the Amalgamverse comics following the Marvel vs. DC crossover, Spider-Boy also sings his own version of the song.
 * One of the first times Deadpool breaks the Fourth Wall is when on his way to fight The Hulk he starts singing the theme song from the Series The Incredible Hulk.
 * In at least one issue of Archie's Sonic the Hedgehog, he sings the themesong for the Sonic Sat AM cartoon that gave birth to the comic. He does the actual lyrics but hums over the part where his own name would be mentioned. "Catchy tune," he notes, "wonder where I've heard it before."
 * Eggman sings his Leitmotif from Sonic Adventure 2 in a later issue.
 * In an early installment of the Mickey Mouse comic strip, "Minnie's Yoo-Hoo" (the character's theme song from the cartoons) is shown playing on a phonograph.
 * An issue of Daredevil has a child singing the theme song to Avengers Earths Mightiest Heroes.

Fan Works
"Phoenix: (That song she's humming sounds really, really familiar...)"
 * Turnabout Storm: Pinkie, in one of her 4th wall breaking antics, starts humming the iconic Pursuit ~ Cornered theme from Ace Attorney in the middle of the investigation.

Films -- Animation

 * In the movie Final Fantasy VII: Advent Children, Loz's cell phone ring tone is the victory music from the game.
 * While The Incredibles doesn't have a proper theme song, the most-repeated motif from the score is called "Mr. Incredible's Theme". Bob Parr can be heard humming this melody when he returns home after a night of clandestine superheroing. In addition, one of the DVD Easter Eggs shows Mr. Incredible dancing to a swing version of the theme.
 * The Hunchback of Notre Dame the melody of "The Bells of Notre-Dame" is played many times throughout the sound track, including in other songs. The tune to "Hellfire" is heard throughout too.
 * Near the beginning of Cloudy With a Chance of Meatballs, Flint hums his own theme music; the exact same music that was the BGM of the previous scene.

Films -- Live Action

 * In Raising Arizona, when H.I. runs into the supermarket from the cops, a muzak version of the movie's theme song is playing.
 * Happens once a movie in the Spider-Man films. In the first one, it's during the montage wherein New York gets to know Spider-Man -- a vaguely stoner-ish guy plays guitar on the subway, singing a song whose notes are those of the Spider Man TV cartoon, though the lyrics are quite a bit more ridiculous. In Spider-Man 2, a busker is twice shown plucking a fiddle and singing the cartoon theme; the movie uses her as a bit of a Greek Chorus whose words haunt Peter. In Spider-Man 3, a parade in Spidey's honor includes the theme performed by a full brass band. One wonders who wrote it in-world and why...
 * Incidentally, Spider-Man 3 is the only one of the three not to have it appearing at the end of the closing credits (the first film has the actual theme from the TV cartoon, the second has a rendition by Michael Buble).
 * The 2003 remake of SWAT has Hondo's team in a bar, singing along with the (instrumental) theme song from the original 70's version. The re-vamped theme finally appears over the end credits.
 * And in the James Bond movie Octopussy, an informant disguised as a snake charmer identifies himself to Bond by playing a bar of Bond's theme music.
 * In the previous film For Your Eyes Only, the code to the room containing the identigraph is part of the tune "Nobody Does It Better", theme tune of The Spy Who Loved Me.
 * In Licence to Kill, Sanchez fires some bullets at Bond and misses. The sound of the bullets ricocheting off the tanker is set to the Bond theme. This apparently was a Easter Egg on the part of the sound editor.
 * In On Her Majesty's Secret Service, there is one scene where Bond walks past a cleaner, who starts whistling the Goldfinger theme, and during the "gadget scene", some old Bond themes can be heard; OHMSS theme can be listened often in the film, such as during the snow chase towards the end. The Bond series seemed to be quite fond of this trope.
 * In Live and Let Die, a New Orleans lounge singer sings a Motown-style reprise of the title theme.
 * In The Man With the Golden Gun, Scaramanga's funhouse features a piano piany version of the theme song.
 * Also in The Man with the Golden Gun, when Bond passes Sheriff J.W. Pepper in a speedboat, a snippet of the theme from Live and Let Die plays.
 * The Thunderball title tune was originally going to be "Mister Kiss Kiss Bang Bang", which does play extensively as an instrumental in the soundtrack -- and is played by a nightclub band.
 * The instrumental theme that appears at the beginning of Ichi the Killer reappears (at least twice) as the ringtone on the main villain's cell.
 * Towards the end of the first Harry Potter movie, Hagrid is playing the series' overture as the protagonists approach his hut.
 * In the third film, when the Boggart turns into a giant snake, the snake theme from Raiders of the Lost Ark is briefly heard. Both films, of course, were scored by John Williams.
 * Taken to its logical extreme in the movie The Long Goodbye where with the exception of the opening and closing of the movie, the only song heard in the movie is various arrangements of the theme song, "The Long Goodbye" used diegetically. So when a character turns on the radio, that's the song that plays, when a character is at a bar there's a piano player singing that song, in the supermarket, a muzak version is playing on the overhead, and when the nudist, hippie, neighbors, are chanting, they're chanting the theme also.
 * In Guillarmo Del Toro's Pans Labyrinth, Ofelia asks Mercedes to sing her a lullaby. Mercedes hums a gentle, mourning tune, which just so happens to be the movie's theme. This lends to the film's complex affair with the boundaries of reality and fantasy.
 * In the live-action Inspector Gadget movie, the theme tune practically haunts the title character -- it's even played by a chamber orchestra at a posh party he is attending.
 * In Brazil, when Sam is entering a code into an elevator, eight notes of the theme song are played.
 * In the 1952 version of Moulin Rouge, the main theme appears as Jane Avril's song in the cabaret.
 * In one of the film's more affecting sequences, Michael Corleone reminisces about his time in Sicily as his son sings an Italian version of "Speak Softly Love (Love Theme from The Godfather)" in Part III.
 * In Ghostbusters 2, Ray and Winston play a tape of the "Ghostbusters" theme song at a kid's birthday party (the ungrateful little yuppie larvae scream for He Man and The Masters of The Universe). The movie also has characters saying the song's line, "Who you gonna call?"
 * In the film Serenity, Jayne is strumming the theme to Firefly on his guitar when the crew first arrives on Haven.
 * In Tourist Trap, one of the secondary cues is played in "music-box" form when a switch is hit, lighting up a display of a wax mannequins.
 * The main orchestral theme of The Ring appears in the actual film as a tune that Samara used to sing as a child. Unsurprisingly, it's Creepy Child.
 * At one point in Dario Argento's Tenebre, a character puts on a record that turns out to be the film's theme music. Another character shouts at her to turn it down, but she ignores the request, and it fades up into the foreground to accompany the following scene.
 * The music that Davy Jones plays on his organ in Pirates of the Caribbean is the same tune as the one played by the music box in his and  lockets. It's also one of the main musical themes of the third movie.
 * The 2008 Get Smart film riffed on the famous opening sequence with Max going through a series of high-security doors with him not clearing one door in time. The theme music stops and he has to input a code, which happens to be the next couple bars of the theme.
 * In ''Back to The Future, when Marty and his band are auditioning to play at the school dance, everybody but Marty plays the opening riff of "The Power of Love" by Huey Lewis and the News -- the theme song for the movie.
 * And his band is promptly shot down by the judge, who is played by Huey Lewis.
 * In Part II, Future Marty tries to play the same tune on his guitar.
 * The same tune can be heard in Part III, on Needles' car radio, when he and Marty are about to race.
 * In Part III, ZZ Top play "Double Back Again" on period instruments at the town dance.
 * When Marty wakes up in Part I after making it back to his own time, the radio plays the end credits theme, "Back in Time," also by Huey Lewis and the News.
 * In the 2008 Live Action Adaptation of Iron Man, Rhodey uses the 90s Iron Man cartoon theme as the ring tone for Tony Stark on his cellphone.
 * And a brass version of the 60s cartoon plays when Obidiah goes to accept Tony's award at the beginning.
 * In Police Academy 5, Proctor is making a paper airplane while humming the movie's main theme.
 * In the remake of Land of the Lost, the theme is repeatedly referenced by lines like "This is the greatest earthquake ever known!" until Rick Marshall sings the show's entire theme song as something he made up. Never mind that he's referring to himself by his last name.
 * In The Star Wars Holiday Special, Princess Leia sings a song vaguely related to Life Day, and vaguely related to the Star Wars main theme tune.
 * Also in Star Wars, it's implied in the Expanded Universe that the Imperial March is the Empire's martial theme. One officer is seen to whistle, "dum, dum, dum, dum-te-tum, dum-te-tum" in a Boba Fett comic.
 * In Rocky III, during the ceremony celebrating Rocky's statue the band is playing "Gonna Fly Now". It happens again at the beginning of Rocky V.
 * At the end of the first Tales From the Crypt movie, Demon Knight, a man implied to be a demon is shown walking down a desert road whistling the series (and the movie's) main theme.
 * In Labyrinth, the Nostalgic Music Box that plays when Sarah picks up the ballroom dancer figurine is "As The World Falls Down", which is played in its entirety later on. Not quite the theme music, but definitely a cameo.
 * The Tribe: Sasha plays the theme from The Tribe on his flute in his first episode as an official Mall Rat. A few episodes earlier, the piece of music Tai San uses during her dance routine is the same incidental music used during establishing shots.
 * The Saint theme, which goes back to the 1939 film The Saint Strikes Back and featured in the 1960's TV show, appears as a car alarm in the 1997 film during an early scene.
 * The Jeremy Brett Sherlock Holmes episode The Creeping Man has Holmes listening to the TV show's theme tune on a music box. Since most readers despise the original short story, this rare bit of levity seems appropriate.
 * Gamera plays his own theme tune on Zigra's back after defeating him in 1971's Gamera Vs. Zigra.
 * In Help, Clang and his thugs attempt to ambush the Beatles in an Indian restaurant, first nabbing the house musicians and replacing them, playing an instrumental medley of tunes from A Hard Days Night (their previous movie) on Eastern instruments.
 * In the live action Thunderbirds movie, Lady Penelope and Jeff Tracy's ringtones are the Thunderbirds March.
 * In the Inspector Clouseau film A Shot in The Dark, the band at Camp Sunshine are playing the film's theme tune.
 * Thomas and the Magic Railroad has Mr Conductor humming the TV show's theme at one point.
 * All the instrumental music in Inception is based on the melody to the Edith Piaf song ("Non, Je Ne Regrette Rien") used to kick the team members out of various levels of dreaming.
 * In the Pink Floyd movie The Wall, the poem that Pink is writing at school is the lyrics of Money.
 * Buckaroo Banzai. New Jersey (Dr. Sidney Zweibel} plays part of the movie's main theme on the piano while the others are trying to break into the Yoyodyne computer system.
 * The Witches of Eastwick (1988) has a very catchy and distinct main theme, composed by John Williams. At one point Jack Nicholson's character is whistling the melody, as if he's able to hear the film's score (this is near the end, when he goes shopping).
 * In the pseudo-Christmas movie Gremlins, as the eponymous creatures carol at the door of the Scrooge-ish Mrs. Deagle, they are singing (read as: screeching off-key) the movie's theme song by composer Jerry Goldsmith.
 * A kind of meta example in the case of Galaxy Quest, in which the in-universe theme song of the eponymous show is a constant presence within the movie's own soundtrack.

Live Action TV
""I can't help thinking I've heard that song before...""
 * In the end of the Get Smart episode "The Not So Great Escape", Control agents whistle the theme song as they leave the Kaos prison camp.
 * Often The Andy Griffith Show would use the music that would eventually become the theme to its After Show Mayberry R.F.D as generic music played for Show Within a Show tv show themes and band music. Mayberry R.F.D. would continue the tradition during its own run.
 * Classic example: In I Love Lucy, one episode has Lucy certain that Ricky has forgotten her birthday. She makes her way to the nightclub where he works, where he assures her he has not forgotten and sings the show's theme song to her, Forgotten Theme Tune Lyrics and all.
 * A second classic example occurred in Green Acres. At the start of the episode, we see Eb going off to do his chores while whistling the theme song. He then turns and looks straight at the camera to say the line "Dahling, I love you, but give me Park Avenue" in Lisa's voice.
 * There's an episode of Boston Legal where Denny Crane has a miniature trumpet, which he plays the theme song on... through the regular part where the theme is played...
 * Also the episode where Jerry hums the theme song through the opening.
 * There is also an episode where Betty White's character's phone goes off, and the ringtone happens to be a snippet of the theme tune to The Practice.
 * In an episode of Mad About You, Paul walks past an a cappella group singing the theme song, which he'd cowritten. To further mess with the wall, he mentions its familiarity. At the end of the episode, Paul and Jamie listen to the same group singing the theme again as the end-credits play. This time, Paul recognizes the tune and grins with satisfaction, but does not identify the tune out loud.
 * Speaking of Paul Reiser, he was also in My Two Dads, which had one episode where the theme song was performed by Evigan (the other Dad of the title) and... special guest Davey Jones of The Monkees.

"JD: Hmm. Catchy!"
 * An episode of Pretty Guardian Sailor Moon has the characters doing karaoke with, and later synchronizing their attacks to, a song by Minako Aino, who later turns out to be Sailor Venus (the song, incidentally, repeats the phrase "C'est la vie", which sounds pretty much like "Sailor V" in Japanese) and which song is available on the soundtrack albums. (All the soundtrack albums. Sometimes more than once.)
 * Tweenies: In one ep, some character has a blow-up guitar balloon and is singing, "Hey, hey, are you ready to play?"... the first line of the show's theme tune.
 * "This Old Man", which in the later years of Columbo was sometimes used as unofficial theme music (the show never really had a theme as such) also appears in a lot of episodes in some form, usually being hummed or whistled by the title character.
 * With a twist. Battlestar Galactica (2005) uses the theme music not from that series, but from the original Battlestar Galactica, as what appears to be the Colonial anthem. In the pilot, an honor guard play it at Galactica's decommissioning ceremony. In the episode Final Cut, it underscores the patriotic ending sequence of a Show Within a Show documentary. And one more time in the final episode when
 * Several main characters on Lost have their own distinct musical motifs. A season 3 episode saw Jack playing a variation of his own theme on the piano.
 * Non-diegetic example: the theme used for submarines, most prominently heard in Season 5, is the cue "The U-boat" from the Medal of Honor soundtrack, also by Michael Giacchino.
 * Stargate SG-1: "Chimera". Carter hums the SG-1 theme music in the elevator, prompting O'Neill to deduce that she has a new boyfriend. Word of God has it that the actress actually wanted to hum MacGyver's theme music as a nod, but she completely forgot the notes so she hummed the Stargate theme instead.
 * The episode "200", which is rife with self-reference, features the hypothetical wedding of Carter and O'Neill. The music in the background begins with Here Comes The Bride, but segues into the SG-1 theme.
 * In a related matter, the SG-1 theme also appears in the BGM of the Stargate Atlantis episode "Home" when the main characters arrive back on Earth.
 * SG-1 returns the favor by using the Atlantis theme in "The Pegasus Project," when SG-1 visit Atlantis.
 * The Arrested Development theme is also Lindsay's ringtone within the show.
 * The opening in the Allo Allo episode "Leclerc Against the Wall" featured the usual opening shot of Rene in his cafe as the instrumental theme song plays. It fades out as Rene begins playing the exact same song on the cafe piano.
 * In another episode an accordionist is heard wandering the town square outside the cafe, playing a slower version of the theme on his accordion.
 * Veronica Mars: At the close of The Teaser to one episode, Veronica wryly notes of an estranged acquaintance, "We used to be friends, a long time ago," the opening words (inverted) of the show's Theme Tune.
 * Similarly: On WKRP in Cincinnati, during one episode recounting events prior to the start of the series ("The Creation of Venus"), Andy explains that he'd come to Cincinnati because he'd "gotten kind of tired of packing and unpacking, town to town, up and down the dial" -- the same explanation offered in the theme song. He lightly lampshades this by saying it as if he were alluding to well-known words.
 * In Fresh Prince of Bel Air, while giving his testimony at the bench during a court case, Will falls into the his lyrics when introducing himself.
 * He Bel-Air'd HIMSELF!? Wha--?
 * One episode of Twin Peaks had the "bumper" music playing on a car radio. Since it was done "cold" right after a break, it had the effect of blurring the line between the credits and the show.
 * Apparently an episode of Twenty Four features the "The Longest Day" remix of the theme tune playing in a club.
 * The Mighty Morphin Power Rangers communicator signal was the original Theme Tune. A Monster of the Week also sings "Go, go, power particles!" to the tune of the theme to taunt the heroes.
 * The Korean Series You Are Beautiful has the title theme not only as background music, but performed by a male singer, a female singer, and a Boy Band during its run.
 * The Sound of Drums that echoes in 's head during the final three episodes of Doctor Who series 3 is the same as that of the theme tune.
 * The serial "Paradise Towers" had a Muzak version of the theme music playing in an elevator.
 * "Vale Decem" in The End of Time is actually being sung by the Ood as the song they promised him. Just as 10 is beginning to regenerate, they shift into a triumphant/heartwrenching version of "The Doctor's Theme."
 * A cello trio is playing Martha's theme in The Lazarus Experiment.
 * Sonozaki Ryubee is seen idly humming his own theme music in Kamen Rider Double.
 * In the finale for season one of Smallville, the band that plays the show's theme song (Remy Zero) is playing at the Smallville High prom. And what do they play? Yup. You got it.
 * When Dr. Swann (former Superman actor Christopher Reeve, of course) tells Clark he's from a place called Krypton -- this being the first time the name is mentioned, a few years into the series -- we suddenly hear one of the fanfares from the Reeves movie series.
 * One episode of the Granada series The Adventures of Sherlock Holmes ends with Holmes beginning to play the series' theme tune on his violin.
 * In Highlander the Source, the Guardian taunts some of the heroes by singing, "Who Wants to Live Forever?", one of the signature songs from the original movie.
 * Midsomer Murders does this all the time -- mostly at fairs or festivals. Mediums playing the theme include a brass band, a youth orchestra, and a marching band.
 * Star Trek:
 * In Star Trek the Original Series, a light cocktail bar version of the theme was played, appropriately, in a bar in "Court Martial" and at a cocktail party in "Conscience of the King."
 * An episode of Star Trek the Next Generation had a string quartet preparing to play in Ten-Forward, and during the warmup the theme tune of the show is subtly played.
 * "Sanctuary" in Star Trek Deep Space Nine, where the instrumentalist Varani plays the theme music for a rapt audience.
 * One Joan of Arcadia teaser has God singing (a humorously bad, given the character) version of Joan Osbourne's "One of Us," which was appropriated as the show's theme song. This segued directly into the credits.
 * The aliens in the War of the Worlds series plot to hide hypnotic messages in a pop tune in one episode. The tune is based on the theme music.
 * In the Growing Pains episode "All The World's a Stage", Mike Seaver and his choir sing the opening theme song -- and it serves as the theme song for that episode.
 * The original title theme of Monk by Jeff Beal, which was since replaced with the good-but-not-as-good It's a Jungle Out There (Randy Newman), has since often come up in the show as background music, usually in dramatic scenes or those having to do with memories. In one memorable instance, however, Randy Disher plays the song on a piano. When Stottlemeyer asks him what he's doing, Disher says "background music".
 * In one episode of Hogan's Heroes, the Heroes pretend to escape so an Italian spy can "capture" them. When he marches them back into camp, they're all whistling the show's theme song.
 * In period drama Upstairs, Downstairs, Sarah performs the stately waltz of the opening tune as a Bawdy Song ("What Are We Going to Do with Uncle Arthur?") in her music hall act.
 * All music on Good Eats is the theme, at different tempi and in different styles.
 * An episode of The Nanny ended with Fran describing how she got to her current station in life by reciting the lyrics of the opening theme.
 * Another episode where a Fran notice similarities between her and a fellow nanny that was going to be fired since the kids had grown up, the other nanny begins to describe that the way she got the job with the opening theme as well.
 * Yet another example was when Fran was describing her fashion sense with "I've got style, I've got flare, that's how I became (Beat) the nanny"
 * The lost culture in the Farscape two-parter "What Was Lost" chanted the series theme as they sacrificed goats. This may have been Foreshadowing the larger role, plotwise and thematically, that they would play in the miniseries.
 * In the episode "Losing Time", John can also be heard humming along to the background music while walking through Moya's corridors.
 * In the Red Dwarf Western-VR episode "Gunmen of the Apocalypse", the piano in the Last Chance Saloon is playing the theme as the main characters enter.
 * In Back to Earth, a haunting rendition of the theme tune is played by doorbell.
 * It's also used as the piece for Rimmer's Skutter organ recital at the end of Dimension Jump.
 * An episode of Lois and Clark has Clark whistling the theme tune.
 * A scene in Scrubs starts with JD and Turk walking in, singing the theme tune (Superman by Lazlo Bane).
 * While not being sung, in a later episode Doctor Kelso is seen talking with some beach babes, saying to them "... I can't do this all my own!"
 * "Guy Love", which only really existed as part of an aneurysm induced hallucination, nevertheless shows up as Turk's ringtone in a later episode.
 * JD and Turk also beatbox the themesong when they're gloating about diagnosing one of Dr. Cox's problem patients.
 * And when Elliot puts one of those electronic "piano carpets" at her door, JD walks in and starts dancing around on it. Guess what he plays.


 * The drumbeat at the start of the theme tune, always played onscreen before it actually goes into the titles, is played by a character on a couple of occasions. In My Office JD plays it out by bursting bubblewrap and in My Best Friend's Baby's Baby and My Baby's Baby it's done by a drummer in a marching band.
 * On Dallas several members of the Ewing family (Bobby, Pam and Lucy) as well as Ray Krebbs went to a disco, where the DJ started playing the Dallas theme tune.
 * Xena: Warrior Princess: Xena played the theme song in one episode (on a lyre that sounded surprisingly like an electric guitar). Another time, she whistles it while fishing.
 * Hercules: The Legendary Journeys also gave us this.
 * Big Wolf on Campus has Merton trying to compose a song on a piano, that is actually the show's theme tune. It's a double reference as the actor who plays Merton created the theme tune in real life. Unlike a lot of other examples of this trope, the song Merton writes actually has vital plot relevance later in the episode.
 * Happens accidentally in Top Gear: the Allman Brothers Band song Jessica, the series' Real Song Theme Tune, comes on the radio while they're filming a "factual review'' of American muscle cars in America.
 * In The L Word, every single tune you hear is some kind of adaptation of the theme music. It's exactly as annoying as you'd think.
 * WindWave Radio in Kamen Rider Double when left on tends to play various bits of music including the eponymous hero's battle themes, all of which were written by fictitious bands from the show.
 * Episodes 21 and 22 has a whistling tune of "Cyclone Effect" -- one of said battle themes - play a part of the case. Philip and Shoutarou sings "Finger on the Trigger" in episode 23 in Fuuuuuutic Idol, their equivalent of American Idol.
 * Another toku example: In one episode of Warrior of Love Rainbowman, hero Takeshi Yamato sings the ED theme, which is his theme song.
 * In the very early episodes of The Beverly Hillbillies, the balladeer not only sings the theme song, but sings new lyrics to reflect the on-screen action, in part a feud between the Clampetts and the Drysdales.
 * In "Jed Throws A Wingding", two old friends of the Clampett family write a song about Jed, which turns out to be the theme song.
 * The two friends are musicians Lester Flatts and Earl Scruggs who provided the music for the theme song.
 * Kate's ringtone is a version of the themesong in a season 2 episode of NCIS
 * Played with on The Big Bang Theory: Howard's cellphone ringtone is the song that was used as the theme tune... in the unaired pilot.
 * In an instance which was bound to happen eventually based off the show's format, Don't Forget the Lyrics offered its own theme song (the Doobie Brothers' "China Grove") alongside the theme of Green Acres in a "TV Themes" category. Unfortunately, the clueless contestant, despite claiming she had never watched it, went with Green Acres, and promptly lost.
 * In The Middleman, the Muzak playing in the lobby of the Jolly Fats Wehawkin Employment Agency (the front for the Middle organization) was based on the show's theme.
 * In Castle, the title character occasionally hums the show's standard "exciting things are happening now" background music to himself.
 * In "Last Call", as he enters an old drinking establishment he used to frequent, the piano man starts playing that same riff. Castle tips him for remembering.
 * The first episode of Three Two One Contact features the recording session for the Opening Theme.
 * In the fourth season finale of House, a cover version of "Teardrop" plays as they try to find a way to.
 * In Greys Anatomy's Musical Episode, Callie sings a line of the show's former Real Song Theme Tune.
 * In one episode of Made in Canada, one of the characters sings the theme song during a party.
 * The New Girl pilot has Jess singing the theme song at one point.
 * In The Wild Wild West TV movie, More Wild Wild West, Artemus Gordon hums the main theme from the show while getting ready in front of a mirror.
 * In the Psych episode "The Amazing Psych Man & Tap-Man, Issue #2", Shawn tells Juliet "I know you know I'm not telling the truth. I know...you know...".
 * The Office (UK) episode "Training" ends with David Brent singing and playing the theme song, "Handrags and Gladrags", on guitar. (Kind of justified in-universe, in that it's a song that predates the series, and presumably was part of the repertoire of Brent's old band Foregone Conclusion, which is discussed in the episode.)

Music Videos

 * The Foo Fighters have done this  twice  three times, amazingly enough, with cameos of prior hits in their music videos:
 * The video for "Monkey Wrench" starts off with a scene of Dave Grohl standing in an elevator, in which a Muzak version of "Big Me" is playing. Naturally, Grohl starts bobbing his head to the tune.
 * The video for "Learn to Fly" shows the airplane that provides the video's setting being cleaned up, to a Muzak version of "Everlong".
 * The song that's playing on the car radio in the beginning of the video for "Breakout" is "Generator", from the same album.
 * The party-crashing bikers from Quiet Riot's "Party All Night" video are first seen hanging around outside a pizza place, with "Bang Your Head (Mental Health)" playing on a boombox.
 * The video to 50 Cent's "I'll Still Kill" has an earlier single, "I Get Money" playing on a TV in the intro.
 * In their Rube-Goldberg-esque music video for "This Too Shall Pass", OK Go includes a shot of a television showing part of the music video for "Here It Goes Again", another of their songs.
 * At the beginning of N'Sync's "It's Gonna Be Me" video-- which is partially set in a toy store-- "Bye Bye Bye" can be heard on the speakers.
 * The Backstreet Boys' video for "Larger Than Life" features an audio medley of the group's earlier singles, presented to sound like someone flipping through several radio stations.
 * At the beginning of George Michael's "Faith" video, "I Want Your Sex" is playing on a jukebox (the very first image of the video). Interestingly enough, upon release "Sex" was used less to advertise the "Faith" album (released in October, as opposed to this song's June) than the recently new film Beverly Hills Cop II.
 * Limp Bizkit's "Rollin'" video began with Ben Stiller and Stephen Dorff pulling up to Fred Durst-- while their car's radio plays "My Generation" (the band's previous single, not the Who song).
 * "Whatcha Say" can be heard at the start of the "In My Head" video, either coming from a car radio or the speakers of the convenience store whose parking lot is the video's location.
 * The opening shot of Alicia Keys' video for "Fallin'" is her bedside radio playing "Girlfriend", another of her songs.

Puppet Shows

 * In Thunderbirds Are Go, Alan is heard humming the theme as he travels on the couch-elevator. Although it's not quite the same principle, in one episode of Thunderbirds the tune "March of the Oysters" from Stingray can be heard emanating from a man's house, and in one of Captain Scarlet the tune "Dangerous Game" from Thunderbirds is on the radio. Not all the radio music was recycled, though.
 * Virgil plays a version of the theme tune on the piano in some episodes.
 * And in the live-action movie Jeff Tracy and Lady Penelope have the theme as ringtones.
 * Many Gerry Anderson series recycle Barry Gray themes from earlier shows. By the time they got to Space1999 almost all of the incidental music was taken from Thunderbirds, Stingray, Captain Scarlet or The Secret Service.

Radio

 * In the first episode of the early-1980s Public Radio serialization of Star Wars, the famous Main Title fades down and is filtered to become -- the background music of a commercial for the Imperial Merchant Marine Corps to which Luke is listening.
 * In Stan Freberg (Creator)'s Dragnet spoof "Little Blue Riding Hood," Granny's doorbell plays the Dragnet theme.
 * In every Dragnet spoof that Freberg made, virtually anything that can make music plays the Dragnet theme -- and nothing else.
 * One episode of The Navy Lark has Sub-Lieutenant Phillips whistling the show's theme music as a "Ditty" he'd picked up somewhere. He's promptly told to stop it by the rest of the cast as it sounds silly and annoying.
 * The Firesign Theater radio show "Nick Danger, Third Eye" has the title character whistling his theme tune as he walks the streets by night.

Theater

 * In the Sailor Moon stage musicals, better known as the Sera Myu, the characters sometimes perform stageshows within the musical itself. One musical has the Senshi, in civilian form, preform La Soldier as part of Rehersal, considered the Sera Myu's theme by the fans (it appears in nearly every musical either as part of it or an encore number), only to have a character complain they'll be recognized by the baddies for singing it. Moments later the baddies show up in Paper Thin Disguises and preform their version of La Soldier (sung by the children in the group). Said scene can be found here.
 * Two examples in the Bernstein/Sondheim musical West Side Story. When Anita arrives at Doc's store, met by the confrontational Jets, one of them turns on the jukebox, which plays a simple reprise of the "Mambo" from the dance at the gym. Then, when Doc goes downstairs to meet Tony, the boy is whistling "Maria" to himself.
 * In A Funny Thing Happened on the Way to the Forum, the characters of Senex and Hysterium sing or hum the songs "Everybody Ought to Have a Maid" and "I'm Calm" at a various points. The first one's a plot point, as Senex mistakes Philia for a new maid (read: concubine), prompting the song. He sings it later while taking a bath, unintentionally leading Erronius to erroneously believe that his house is haunted.
 * In Assassins, John Wilkes Booth is heard humming a bit of "The Ballad of Booth" before talking with Lee Harvey Oswald.

Video Games
"Cloud: I've heard this somewhere before. Managed to play well."
 * In Luigis Mansion, Luigi hums or whistles along to the background music.
 * If you look at the map in dark rooms (or just listen very closely), you can hear the ghosts singing along with the background music as well.
 * The Game Boy Horror's "ringtone" is a 16-bit version of the same music.
 * In the Music Room, all the instruments can be individually activated to play an orchestral arrangement of the Super Mario Bros theme song.
 * The subspace (behind-the-door) world in Super Mario Bros. 2 plays the first game's theme. The Super Mario All-Stars version of SMB2 reuses SMB's corresponding version.
 * The music box in Super Mario Bros 3 plays the original Super Mario Bros theme song.
 * At the begining of Mario & Luigi: Superstar Saga, Mario is heard humming the Super Mario Bros theme while in the shower.
 * Paper Mario the Thousand Year Door uses the title theme from Super Mario World to let you know Peach has sent an e-mail.
 * The game also uses the ice world theme from Super Mario Bros 3 when you receive a message from a mysterious informant, "Mr. X".
 * Anything in the Paper Mario/Mario & Luigi series that would logically play music WILL play something from Mario's old days. Case in point: the record player in Boo's Mansion from the first Paper Mario, dust and all.
 * And Mario will whistle his original theme music in Super Mario RPG when he hops in the shower at the inn in Marrymore.
 * In the Hilarious Outtakes for Mario Power Tennis (soon to be given an Updated Rerelease on Wii), the Super Mario Bros theme plays as a ringtone -- on Wario's cellphone, of all things!
 * It is also a ringtone for a cell phone item in Mario Party 3.
 * In Super Mario 3D Land, Mario hums the classic theme music after receiving the picture between Worlds 5 and 6.
 * In Tomb Raider: Anniversary, upon hearing a music box play the Tomb Raider main theme, Lara comments "What a lovely tune!"
 * In Grand Theft Auto III, the ringtone in Claude's pager is the opening of the song "Grand Theft Auto" from the first, top-down GTA.
 * This reappears in Grand Theft Auto IV as one of the selectable ringtones on Niko's cell phone.
 * In the game Psi Ops the Mindgate Conspiracy you find an elevator which is playing a muzak version of Cold's "with my mind" (A song made for the game).
 * In the videogame Trapt, the first "Dark Illusion" (a kind of particularly gory and lethal traps inset in the enviroment) that the player is likely to find, is "The Man-Eating Music-Box" -- which plays the game's "Melancholy Theme" as it crushes the helpless victim within its gears...
 * In Suikoden V, a group of Dragonhorse Knights plays the game's main theme on flute, as part of an 'Ancient Ritual'.
 * In Castlevania 64, Malus plays a haunting song on his violin. The song is actually Bloodlines from Rondo of Blood with a slower tempo.
 * The ringtone of Godot's cell phone in Phoenix Wright Ace Attorney: Trials and Tribulations happens to be the same music as his own Theme Tune.
 * cell phone ringtone in Apollo Justice: Ace Attorney is one of Klavier Gavin's band's songs, Guilty Love, sped up to almost double the tempo.
 * Klavier himself plays a recording of Guilty Love by way of an introduction in court. It's somewhat safe to assume that every single repeat of the theme tune afterwards is just Klavier toeing "play" on the CD player beneath his bench.
 * Even Phoenix's cell phone is like this: his plays the Steel Samurai theme song.
 * In case 1-5, Ema Skye's cell phone plays the Steel Samurai theme song as well. This allows Phoenix to pass it going off in the parking garage as his when the detective at the scene asks what just happened.
 * One of the characters in Wild Arms 5 has the first bars of the theme of Wild Arms 2 as a ringtone.
 * Characters using the /bored emote in World of Warcraft will sometimes whistle a few bars of the in-game music.
 * Portal includes radios that play a very perky, jazzy version of the end credits theme "Still Alive".
 * At the very beginning of No More Heroes Travis whistles the game's theme.
 * At the weapons shop in Xenon 2, the alien shopkeeper is listening to the theme tune. He turns it off to serve you.
 * At the beginning of Persona 3, the Main Character listens to a remix of "Burn My Dread," the opening theme, on his headphones. A different remix is also used as background music in the very last battle of the game. "Burn My Dread" is also heard in the first episode of the spinoff anime Persona ~trinity soul~, playing briefly just before Igor makes his cameo appearance.
 * As well, the actual melody of the main theme appears in no less than six songs on the game's soundtrack.
 * Max Payne 2 has a janitor singing the game's theme "Late Goodbye", along with one of the enemies playing it on piano.
 * In both Max Payne games, using the piano will have Max play a rather unskilled rendition of the main theme song. He does a better job the second time you try. Guess it's secretly an RPG.
 * In the King's Quest Fan Sequel The Silver Lining, clicking the "talk" icon on Graham will cause him to whistle a variety of tunes which would be well-known to adventure game fans, including the King's Quest theme, the Space Quest theme, the Leisure Suit Larry theme, and the Monkey Island theme.
 * Duke Nukem 3D has several references to the main theme, "Grab Bag" -- Duke whistles it during the end of the second episode, and it also reappears in muzak form in the Atomic Edition level "Shop & Bag".
 * Inverted in Final Fantasy VI; the first time in the game that Celes's theme is heard (indeed, hers is the only one that doesn't play during the rename screen) is when Celes sings it in an opera.
 * Also, in Dissidia Final Fantasy Kefka sometimes hums the Victory Fanfare when he wins.
 * In Final Fantasy X, the chocobo caretaker in the SS Liki ship sings the chocobo's theme a bit when trying to talk to her after the cutscene.
 * In Final Fantasy VII, Cloud must play a tune on the piano in Tifa's house in order to obtain her ultimate limit break. It turns out to be the game's main theme, and is vaguely familiar to Cloud...


 * In Final Fantasy VIII, the game's end theme, "Eyes On Me," is a hit pop song in-game and is first heard as an instrumental piano piece played at a nightclub by Julia Heartilly, who is soon inspired to write lyrics for it thanks to Laguna's influence.]
 * The game's end theme in Final Fantasy IX, "Melodies of Life," is also important in-game. Zidane first hears Garnet/Dagger singing it on "la" in Dali and later in Lindblum. He asks her what it is and she admits that she doesn't know, and can't even remember where she learned it, but singing it comforts her when she feels sad or lonely. Zidane says that it must be a mystical song. He later hears her singing it in Dali and when she stops, he tells her that it's okay for her to kee singing "'Cause it's our song, right?" As they float out to sea on a boat, they hear the song coming from the Eidolon Wall and hearing it causes Garnet/Dagger to have a flashback and remember some of the details of her early childhood. Finally, at the end of the game,
 * In one scene early on in The Legend of Zelda Twilight Princess, Midna hums her own leitmotif to herself.
 * The Legend of Zelda Ocarina of Time uses this as well. A portion of the soundtrack includes extended versions of songs Link learns throughout the game, Malon sings in time with the ranch's background music and Ganondorf plays his own theme tune on a pipe organ.
 * In The Legend of Zelda Majoras Mask when visiting the rooms of the Indigo Go's each member is playing a different song from The first and second games: The Pianist plays the Game Over Screen from The Legend of Zelda, The Guitarist plays The Dungeon Theme from the same game, and The Drummer plays the Small Dungeon Theme from The Legend of Zelda a Link To T He Past.
 * The Legend of Zelda Skyward Sword has
 * At one point in Metal Gear Solid 4 you have to follow a man disguised as PMC soldier. How do you recognize him? He is whistling the game's theme tune. (Or you can be uncreative and use the Solid Eye).
 * The Episode VI Jabba's Palace level in Lego Star Wars II features radios that play a rock version of the Imperial March. Bonus: Headbanging Gamorreans.
 * This song was originally featured in Star Wars: Force Commander, an old PC game. Entitled the "Rage Mix", it was once available for download, too. An archive of that page is here.
 * You can also hear the disco version of the Star Wars opening theme in the first game by completing a quick side puzzle on Kamino.
 * In Star Wars Republic Commando one of the elevators on a Republic ship has kind of a muzak version of the Imperial March playing in the background. Fortunately, it's accessible in the game's files.
 * In Star Wars Battlefront II, Stormtroopers will occasionaly whistle the Imperial March.
 * The protagonist's ringtone in Time Hollow is a MIDI version of the main theme.
 * A trailer for Sonic Unleashed (also included as part of an intro video in the game itself) begins with Sonic whistling the tune from Sonic 3's Angel Island.
 * In Deus Ex, using a piano will play a few notes of the opening theme.
 * In Deus Ex Human Revolution music from the first game can be heard on radios and whistled by NPC's.
 * If you activate the piano in the second level of James Bond Nightfire, he will play the James Bond theme.
 * In Time Splitters Future Perfect, there is an organ in the second level. One of the songs are the theme song from Anaconda, a minigame in Time Splitters 2.
 * During a certain event in Persona 4, Burn My Dread, the opening theme from Persona 3 is playing as a video in the background.
 * In-universe example of a sort: if you talk to Teddie outside Mitsuo's video game themed dungeon, he complains that the music from it is stuck in his head.
 * In Space Quest VI, Roger will whistle his own theme song if he stands still long enough.
 * Devil Survivor's Haru frequently performs "Reset", the game's opening theme. The instrumental version is practically her Leitmotif.
 * The Simpsons Game contains a puzzle in which the player must guide Homer or Bart over FAO Schwartz-esque floor tiles to play the first few bars of the show's theme song. Bart reacts to the puzzle's solution by shuddering, "That tune...it's been haunting me for years!"
 * This also serves as a Call Back, since one episode (season 5's "Lisa vs Malibu Stacy") ends with Homer doing pretty much the exact same thing.
 * At the end of her concert in Mega Man Star Force 3, Sonia Strumm performs a new song she's composed called "Shooting Star," which happens to be the game's opening theme.
 * In one of the Hot Springs events of Tales of Symphonia, the female characters are singing the game's opening, "Starry Heavens". It's of course Lost in Translation in the US and European versions as the opening J-pop song is replaced by an original instrumental work.
 * All of the intro movies in Street Fighter IV include that character's theme playing in the background.
 * In the third Monkey Island game, you can talk to Edward van Helgen about a horrific sea voyage where he and his crew were haunted by a diabolical, cacophonous melody coming from the wreck of a ship they had visited. Most of his crew went mad and threw themselves into the sea. When Guybrush asks him how the tune went, van Helgen will hum the classic theme to the Monkey Island series.
 * Mega Man 7 contains an example -- in the introductory cutscene, Mega Man is helmetless and is riding in a truck to the intro stage. When he puts on the Metool helmet, a short theme with an abrupt, dissonant Record Needle Scratch-esque ending plays. Mega Man then dons his iconic blue helmet, and the recurring "Robot Master Selected" theme plays.
 * In Fallout 2, a mentally-retarded janitor can be found singing (with a considerable stutter) the song Maybe, the theme from the first game. It's a place to boost your Karma Meter by either praising him for it, or telling him to shut up.
 * While a bit of an Easter Egg, each of the buttons on the main screen in the Peggle series makes a little "bloop" noise when the mouse rolls over it. These aren't random; playing the closest thing to a theme that Peggle has (Ode To Joy) produces a little congratulations scene.
 * Pokémon Gold and Silver have Team Rocket hijack the radio tower as part of their plan to find their missing boss, Giovanni. The music that plays on all radio broadcasts while they do this is the same theme that plays when a Rocket Grunt challenges you to a battle.
 * The alien shopkeeper who sells you weapons in Xenon 2 is listening to a tinny version of the game's theme tune when he appears, then turns it off to do business.
 * In the elevators in Mass Effect, one of the tunes piped in is a muzak version of the game's main theme.
 * In Halo 2, the Prophet of Regret holograms chant the series' theme tune.
 * The elevators in Golden Eye 1997's Control Center play a muzak version of the Bond theme.
 * Several characters in the Ys series have their Leitmotif played on an instrument, eg "Feena" on harmonica, "Niena" on ocarina, and "Olha" on flute.
 * At one point in Heavy Rain, Norman Jayden plays his own theme song as a Lonely Piano Piece.
 * In an old DOS game called Normality, the opening Cutscene shows the protagonist being arrested for whistling. The tune he was whistling was the game's theme tune.
 * Partial example: The Mega Man X series uses a rocking metal cameo of the musical stinger that plays when you pick a robot master in the first Mega Man game.
 * At least one song in every Silent Hill game after the first includes a part of the track playing notes from the original game's theme. Namely Theme of Laura, Hometown, One More Soul to the Call, and Hell Frozen Rain.
 * The Pikmin will hum the first few bars of their theme tune when traveling.
 * An Easter Egg sees the Pikmin hum the song used in Japanese advertising for the game, "Ai no Uta," in-game.
 * Although a Dolled-Up Installment, Super Spy Hunter has a remix of the Peter Gunn theme (aka the original Spy Hunter theme) in its second stage.
 * The lyrics to "Dragonborn", the theme music of The Elder Scrolls V Skyrim, can be found in an in-game book, Songs of Skyrim, along with a translation.
 * In the Japanese arcade game, Super Sentai Battle: Dice-Oh, Dragon Ranger plays a bit of the game's theme song on his Zyusouken while using his special attack.
 * Conker of Conker'sBadFurDay is apt to whistling the theme to Windy & Co. when left idle in the main area of the game.
 * Conker also whistles his theme tune during the It's War chapter to pass the time after waiting for an electric chair to finish frying its patron. As seen at 6:47 in the following clip
 * Windy theme's also heard in the radio during Nuts & Bolts intro.
 * Quintessence the Blighted Venom: The opening song (To Realize) is not only played in-character, but exists in universe before hand, and has some plot significance. (Not much, though some.) And played by the main characters; one using an ocarina (Reivier) and one using a piano. (Lunair) Sadly not at the same time.

Web Original

 * In Act II of Doctor Horribles Sing Along Blog, the Doctor's cellphone ringtone is the first two bars of the Bad Horse theme.
 * Mr. Deity's cellphone plays the Mr. Deity theme song.
 * Used as a Mythology Gag in episode eight of the The Joker Blogs: Joker hums his theme tune from the animated series while
 * The ending credits music of Suburban Knights can be heard over the stoner's radio in the first scene.
 * In the first scene of the first episode of RWBY, Ruby's listening to "This Will Be The Day" on her player, before it's even been used as the theme tune for the first time.

Western Animation
{{quote| Casper: Oh, great. Now I've got that song stuck in my head.
 * In House of Mouse various characters have a Leitmotif that plays in the shorts and their intros. Several episodes feature Mickey humming along with his own theme.
 * I Am Weasel, "I are Music Man". Red guy repeatedly sings the lyrics of the show's theme song to "Pop Goes The Weasel" (which is in fact the melody of the theme).
 * Popeye used to sing his theme song in every cartoon in the really old days, and, as time went on, he still contented himself with humming or scatting the theme of the tune while walking down the street.
 * In the Powerpuff Girls episode "The Powerpuff Girls' Best Rainy Day Adventure Ever," the girls stay inside and pretend to be themselves, humming their own theme song. One of only a swarm of hilarious Mythology Gags and more normal jokes from that particular episode.
 * In The Simpsons episode "Bart Gets Famous", Bart comes downstairs whistling the Simpsons theme song only to be reprimanded by Marge for whistling "that annoying tune." (This is the first of many self-referential jokes within this episode.)
 * The producers of Simpsons were thinking about adding a groan / scream to the beginning of the theme, simply because the odds that someone would be groaning at hearing it again.
 * In "Angry Dad - The Movie", the theme song is played when Bart wins an award.
 * The same thing happened with Homer in "Brother Can You Spare Two Dimes".
 * One Casper Christmas special features the title ghost meeting up with a girl named Holly Jolly (yes, really). Holly, embarrassed by her, ah, seasonal name, tells Casper to imagine how embarrassing it would be to have "Holly Jolly" for a name -- Casper proceeds to imagine his own theme song, with the words "Casper" replaced with "Holly Jolly". Its Ear Worm tendencies are also lampshaded near the end of the scene:

''(flies off with the "Holly Jolly the Friendly Ghost" song playing in his head} }}
 * Kim Possible uses this in several ways. The Kimmunicator's ringtone is a distinctive four-note sequence that's not only in the theme song, but is emblematic of the series. In one episode, the team faces a group of professional heroes for hire with their own theme song, and Ron spends the rest of the episode writing one for himself and Kim. He comes up with, of course, the show's theme song.
 * Either Kim or Wade must be a fan of Jack Bauer. Listen to the Kimmunicator, then the ring tone in 24, and tell me they're not similar. Go on, do it.
 * Also in 'Sitch in Time' she gives Ron his own "Ronmunicator" so he can "Call me or beep me, you know, if you want to reach me." Plus, in So the Drama, Kevin's dad says he'll give Kevin her number, so he can "call you, bee-" (and that's as much as we get, but we all knew where it was going).
 * And in another episode, Ron attempts to win a rap music contest, and his entry is Rufus's theme from the soundtrack.
 * Ron also hums the "Naked Mole Rap" in at least two other episodes.
 * Pinky is knocked silly in an episode of Animaniacs and mumbles "There's baloney in our slacks" as he stumbles around. It's a line from the opening theme.
 * Another episode has Wakko playing with this while singing to himself: "We're tiny, we're toony, we're all a little looney... Nah, I'm tired of that one. It's time for Animaniacs!"
 * And another episode featured Beethoven attempting to compose his ninth symphony. Songs he tries and rejects include the themes of Looney Tunes, Tiny Toon Adventures, and Animaniacs. Apparently the writers liked this gag.
 * Teen Titans:
 * In one episode, a happy, upbeat Cyborg sings a version of the theme song centered around himself.
 * In Teen Titans Trouble in Tokyo, Beast Boy does a karaoke version with surreal Gratuitous English lyrics, and the entire cast sings this version over the end credits. Weirder still, the oddball lyrics Beast Boy sings are a mostly-accurate English translation of the Japanese version of the theme song!
 * The Titans' communicators play a tinny, ringtone-style version of their title theme's primary lick.
 * The Charlie Brown and Snoopy Show: There's a skit surrounding jack-in-the-boxes, each of which plays the show's theme song. In another skit, Snoopy plays an organ grinder playing the theme tune as well.
 * Charlie Brown's Christmas Tales has Snoopy playing a little music, only for Lucy to pass by and comment that "O Suzanna" wasn't very Christmas-y. In response, Snoopy switches to "Christmastime is Here", from the original A Charlie Brown Christmas.
 * At the end of the This is America, Charlie Brown episode on American music, Charlie Brown is asked his favorite song, and he replies that it's one by Vince Guaraldi. "I believe it's called 'Linus and Lucy' -- by coincidence."
 * Classic example: The Smurfs are prone to la-la-ing their own theme song.
 * In an episode of Arthur, while Binky is giving advice to D.W., he begins to recite the lyrics of the the show's theme song. When the familiarity is pointed out, he comments that he heard it somewhere but can't quite remember where.
 * Another episode has an instrumental of the theme being played off a CD. Arthur comments it sounds familiar, and Buster wonders if the music contains subliminal messages.
 * Transformers has done this on a few occasions:
 * Hardhead sings the ending theme at a karaoke party in the Transformers Headmasters episode "The Shadow Emperor, Scorponok".
 * Kakuryu sings a bit of the end theme to Transformers Victory in an early episode. Considering the theme tune sings about driving villains away, and, this qualifies as Foreshadowing.
 * Beast Wars features Waspinator, after being shot out of the air, warbling a line from the original Transformers theme tune just before he collapsed into a crumpled heap.
 * In the pilot episode, when Cheetor encounters Waspinator, he first thinks it's a real wasp. When he realizes it's not, he says it must be "Robots in Disguise!", the famous line from the original Transformers theme.
 * Animated once showed Sari humming the theme song while listening to some music on headphones, even though the music playing sounded different. In another episode of Animated, Bumblebee left an auto parts store with a car horn that played the first few notes of the theme song. An in this short, Ratchet whistles it.
 * On A Pup Named Scooby Doo, at the end of an episode, features two pop stars singing their new song, the theme song of the show.
 * One of the Scooby Doo movies has Velma singing the classic theme song for a concert.
 * On What's New Scooby Doo Shaggy has the theme as his ringtone.
 * Almost every time recurring characters The Hex Girls (a Wiccan rock band) show up on What's New Scooby or in the movies, they end up singing the theme song.
 * In one episode of Scooby Doo Mystery Incorporated, Daphne hums the theme of The New Scooby Doo Movies while brushing her hair.
 * Scooby Doo On Zombie Island had Fred hum the Scooby-Doo, Where Are You? theme song while getting ready for dinner.
 * Often done on Fosters Home for Imaginary Friends, by various characters.
 * In an episode of Justice League Unlimited, Green Arrow hums along to his own Theme Music Power-Up (though it's his personal Leitmotif, not the show's theme). And then the instrumental version plays, similar to the Castle example above. Link.
 * Earlier in the DCAU, an episode of Batman the Animated Series had the Joker whistling his own Leitmotif.
 * In the episode "The Once and Future Thing Part 1", when the time-displaced Justice League members saddle up with some of DC's other Old West characters and take off to the villain's hideout, a Western version of the original Justice League theme plays.
 * Coincidentally, the "Western Justice League" consists of seven members, the same as the show's original League.
 * Garfield and Friends often had characters humming or softly singing the theme songs. Interestingly, this they'd continue to sing the original theme even after the show had switched to its second theme song; possibly because the new one, while less inappropriately saccharine, was not as easy to hum.
 * Lanolin even parodies the theme in one episode:
 * Orson would eventually sing the second theme song in an episode from the fifth season.
 * At the end of an early episode of X-Men Evolution, a relaxed, jazzy tune plays over the kids' enjoying themselves in the pool. It's a version of the early '90s X-Men cartoon theme.
 * Occurs in The Fairly Odd Parents and later Danny Phantom, both with cell phones.
 * In the former, it also occurs when Timmy wishes to be able to read minds, and hears elevator music when reading Cosmo's.
 * The Jetsons' door bell plays the first 4 notes of their theme song ("Meet George Jetson...").
 * A more comedic episode of Gargoyles ends with a minor character walking away, humming the show's theme.
 * The Drawn Together theme appears in altered versions serving as a sub-theme in various episodes.
 * In "Ghostesses in the Slot Machine", a jazzy version serves as the theme to a Trading Spaces parody.
 * In "A Very Special Drawn Together Afterschool Special", a mellow version serves as the theme to a parody of Very Special Episodes.
 * In "Mexican't Buy Me Love", a tejano version serves as the theme to a Mexican-flavored episode.
 * In an episode of Freakazoid, Dexter hums a few notes of the opening sequence as he takes out the garbage.
 * Jake and Halley's cellphones in American Dragon Jake Long feature the opening theme as a ringtone. Spud also sings a few bars of the themes in one of the later season 2 episodes.
 * Avatar: The Last Airbender has a slightly more dramatic version in "The Firebending Masters" when the closing theme is played by characters during the climax of the episode.
 * In an episode of Care Bears: Adventures in Care-a-lot, Cheer, Harmony, Trueheart, and Love-a-lot start a band and are heard practicing the show's theme song.
 * In an episode of Darkwing Duck a brief scene shows Megavolt pacing in his lighthouse tower, singing a version of the title theme, with no lyrics. He soon breaks off and complains, "Agh, I can't get that song out of my head!" His irritation is understandable, since the song is all about his nemesis.
 * In The Spectacular Spider-Man,
 * On his way to Hobo 13 in the episode of the same name, Invader Zim shows off to GIR while flying the Voot Crusier. While doing so, he starts sing the theme song.
 * At the beginning of the Dungeons and Dragons cartoon episode "Child of the Stargazer", Diana is humming the D&DC theme-music.
 * One episode of Chowder had Shnitzel literally facing the music, which happens to be the show's theme song. Chowder cheerfully hummed along while Shnitzel begged them to stop.
 * Done in another episode when the tune plays on the radio and Mung Daal sings along.
 * In the Animated Adaptation of the British comic strip Rupert, the titular character once whistles his Instrumental Theme Tune as he walks along the beach.
 * Undergrads has Good Charlotte guest star as the band preforming at a fair State U is having. Nitz says something along the lines of "Good Charlotte? What have they done that I've heard of?" Someone immediately walks by, listening to the show's theme song on an MP 3 player. The band later sings the song at the fair itself.
 * An episode of Horrid Henry has the titular character sing the theme song to a group of OAPs. He says that it's an "old classic" of "The Killer Boy Rap Stars" (Henry's favourite band).
 * In Caillou's Holiday Movie, Caillou performed an original Christmas song to the tune of the show's theme song.
 * One episode of Megas XLR has Coop teaching an S-Force friend how to sing Karaoke. We don't get to hear the music, but we do see that the lyrics are the show's theme tune.
 * Not a theme tune, but in the Veggie Tales episode "Madame Blueberry," the episode's Silly Song ("His Cheeseburger") shows up as muzak playing in the food court.
 * Done as a deliberate fourth-wall breaker on Samurai Jack, where a boy whose family Jack stays with overnight expresses his admiration for the famous roving hero by singing a line from the show's theme song.
 * Several characters have whistled the theme tune for Total Drama Island, most notably Chef in "Paintball Deer Hunt".
 * And in the third season, it's revealed Chris's ring tone is the theme.
 * In Wakfu, characters sometimes whistle the opening theme song. Ruel even hums it and references some of the lyrics. A big example of this trope is when Ruel uses a remix of the theme song as the sequence to a dance-pad activated lock to his house right afterwards. Evangelyne also hums it while showering in episode 22.
 * The Penguins of Madagascar has its theme as elevator music and one Kowalski whistles it.
 * The end of the Family Guy episode "Believe It or Not, Joe's Walking on Air" had Peter humming the theme song (with the line "Joke's on you" in place of "Laugh and cry" included) when Joe is back to being paraplegic.
 * The 2009 version of Strawberry Shortcake has the theme song as a ringtone for her cell phone.
 * Some of the lyrics of The Boondocks theme tune make a cameo in "The Story of Catcher Freeman", specifically "... [be] a soldier, until the war is won".
 * In a few episodes of My Little Pony Friendship Is Magic, characters can be heard humming the first line of the theme song, the same jingle that appears in EVERY COMMERCIAL IN THE WHOLE TOYLINE.
 * Specifically, Spike in "Griffon the Brush-Off" and Fluttershy in both "The Ticket Master" and "A Bird in the Hoof".
 * Actually in "The Ticket Master" Fluttershy hums the theme of G1's My Little Pony.
 * During the episode "May The Best Pet Win!", the bat competing to be Rainbow Dash's pet plays the theme tune on a set of water glasses.
 * The traditional My Little Pony theme occurs often on Friendship Is Magic, mostly as background music, although Twilight Sparkle has hummed it.
 * In Fish Hooks, many of the characters have the theme song set as their ringtone, most notably Bea.
 * One episode of Trapdoor had Berk finding a boombox and playing it. What played from it? The full version of the theme tune! The entire episode being nothing more than a glorified music video.
 * Phineas and Ferb has an odd example. While the song doesn't come up, the stuff mentioned/shown in the theme does. They've checked "finding a Dodo" off of a list with several things mentioned in the theme, Buford one time complains that they should have given a monkey a shower (which they actually did early on), and "Painting the Continent" appeared in one of their first episodes ever.
 * They eventually make up for this in the Big Damn Movie - namely, the song "Summer (Where Do We Begin?)" briefly segues into the opening theme.
 * In the G.I. Joe episode "Cold Slither" COBRA tries to brainwash audiences with subliminal messages in a rock band's music; the Joes foil the plot at a concert, and pacify the crowd by becoming The Average Joe Band. Guess what song they play?
 * In the Galaxy Rangers episode "Battle of the Bandits" our heroes perform (or pretend to perform) the theme song in a competition.
 * Maryoku Yummy, in the episode "The Best, Best Friend".
 * The opening scene of Cosgrove Hall's The Wind in The Willows (the feature-length adaptation of the book, rather than the later series) has Mole humming the theme tune as he does his spring-cleaning.
 * The familiar themes to Merrie Melodies and Looney Tunes have been featured in several early cartoons with lyrics. "Merrily We Roll Along" was used in Billboard Frolics (1935) and Toy Town Hall (1936, both performed by an animated Eddie Cantor, who co-wrote the theme) before even becoming the Merrie Melodies theme. "The Merry-Go-Round Broke Down" (Looney Tunes) was a standard from 1934 and was performed by Daffy Duck with different lyrics in 1938's Daffy Duck And Egghead.
 * In at least one episode of Alvin and The Chipmunks, the theme song popped up on the radio.