Theme and Variations Soundtrack

A soundtrack trope. Basically, the application/adaptation of the musical form Theme and Variations to a soundtrack. Alternatively, this is the trope Recurring Riff, But More—enough to basically dominate the soundtrack. This is when you hear said recurring riff all over the soundtrack; many of the tracks are effectively variations on this "theme" of the work.

Happens slightly more often if the work has a catchy theme song motif that can be worked into the soundtrack.

This trope is also like a highly elaborate and very much expanded version of Variable Mix.

Anime and Manga

 * 5 Centimeters Per Second: Most of the music in the film is an arranged version of either the piano theme from the trailer, or of "One More Time, One More Chance"
 * Elfen Lied: Arrangements of "Lilium" (the opening credit theme) are used frequently as background music, covering scenes with wide arrays of emotions such as sadness, nostalgia, serenity, suspense, and murder.
 * Mobile Suit Gundam 00: About half the tracks are remixes of one theme. Most of the other themes in the series get remixed a few times.
 * Likewise Gundam Wing, where the most prominent is Heero's battle theme "The Wings of a Boy Who Killed Adolesence", but done subtly; sometimes the only tip-off is that riff of nine notes from the start.
 * Stratos 4: A number of tracks, including one labeled "Mikaze's Theme" (for the lead main character), that all share a tune. At the last episode, this is revealed to be the tune of the second ending theme (which is used for the ends of whole seasons).
 * Voices of a Distant Star: In the soundtrack notes, Tenmon remarks that he essentially created the soundtrack by writing a "Theme A" and "Theme B", and then made variations on those two themes to provide appropriate background music for every scene.

Film

 * in Titanic, most tracks that don't play during the iceberg collision or the sinking are mostly variations on either the "Southampton" Theme or the Love Theme.
 * In the German adventure movie F. P. 1 antwortet nicht (1932), composer Allan Gray (born Josef Zmigrod) makes good use of the main theme. In the course of the film you get to hear it as the song "Flieger, grüß' mir die Sonne" (Aviator, greet the sun for me) sung by the hero (Hans Albers in German, Conrad Veidt in the simultaneously produced English version, Flying Platform 1 Does Not Answer, and Charles Boyer in the French), as a march, as a foxtrot, a waltz, and in various pieces of incidental music. The song became a hit again in the 1980s in a cover version by the band Extrabreit.
 * The soundtrack to Pan's Labyrinth. Fortunately, "Mercedes' Lullaby" is also a Crowning Music of Awesome.
 * How to Train Your Dragon.
 * Inception had its score built around orchestral variations on Edith Piaf's "Non, je ne regrette rien".
 * The recurring themes of Honey, I Shrunk the Kids are "Strange Neighbors" and the main title.
 * You Never Dreamed bases its short score on "The Last Poem".
 * The Fountain's score uses several variations of "Death is the Road to Awe."
 * Paul McCartney's soundtrack to the 1966 film The Family Way largely consists of variations on the main theme. It is lovely, though, to the extent that McCartney had it played as he walked down the aisle (the second time).
 * Just about any Irving Berlin film, like Holiday Inn or White Christmas, because by contract only his music could be used them. Consequently, the melodies of the various musical numbers will be found weaving in and out of the background music.

Live Action TV

 * Good Eats does this about three times or more per episode with the theme song.
 * The theme song to Jeeves and Wooster is jazzy and upbeat. The episodes manage to make the tune span everything from sentimental to sad to sinister.
 * Great British Menu. All the music is based on the main theme, but there are something like 300 versions.

Theatre

 * Wicked does this with the chord progression and parts of the melody of "No One Mourns the Wicked", particularly the Overture. This theme is actually based on a piece from the Rock Opera The Survival of St. Joan, in which Stephen Schwartz was musical director.

Video Games

 * Final Fantasy XII is supposed to have one particular piece that recurs in almost every track in the game.
 * Half of Final Fantasy XIII's soundtrack is based off of one song, "The Promise."
 * The Legend of Zelda: Twilight Princess does this with its main overworld theme.
 * And to a lesser extent, The Ballad of the Goddess from Skyward Sword is based on Zelda's Lullaby reversed.
 * The Mario series and its spinoffs tend to like this a lot.
 * Super Mario 64 has that iconic main theme that's first heard in Bob-Omb's Battlefield. The entire theme is remixed for the snowy mountain and "slider"/Rainbow Ride themes, and its motifs are used in the level complete, game over, and intro sequence tunes.
 * In New Super Mario Bros. Wii, pretty much every track (besides the final levels) that isn't a reference to an older Mario tune is a rearrangement of the regular Overworld theme.
 * Super Mario Land 2: 6 Golden Coins has the main theme in some form in almost the entire soundtrack.
 * Super Mario Land 3: Wario Land also does this, but to a lesser extent than SML2.
 * In Super Mario Sunshine, Delfino Plaza's BGM is rearranged for, again, nearly every overworld stage.
 * Yoshi's Story also has its main theme in many of its tracks.
 * As does Yoshi's Island DS.
 * Super Mario World was the first in the series to do it; every level theme was a variation on the same tune.
 * The music for the "Wild Glide Galaxy" and "Freezy Flake Galaxy" levels of Super Mario Galaxy 2, which are both remixes of the music for the "Sky Station Galaxy" level, as with the game's title and ending themes and the bonus rooms.
 * In Mother 3, much of the music is based on the Pigmask Army's theme, being heard more and more often as Porky's influence spreads. Several other variations of other tracks can be heard throughout the game, too, even if the events in which they play have no relation to one another, such as the hot spring theme being a slower version of the track that plays when Wess dances to open the door in Osohe Castle, or how the theme of the Tazmily hotel (post-Time Skip) is rearranged for the cinema in New Pork City.
 * Punch-Out!!! Wii takes the Punch Out! theme and culturally rearranges it for every boxer.
 * Tower of Heaven follows this trope to a T.
 * The music in Ultima Underworld II was a set of variations on a four-bar Phrygian progression theme.
 * Much of the music in Ace Attorney Investigations features Miles Edgeworth's theme, The Great Revival.
 * In The 7th Guest, most of the music pieces, including the characters' leitmotifs, are variations of "The Game".
 * Most of the music in Ikaruga is based on "Ideal", the first chapter's theme. Pretty much the only music that doesn't use this theme is "Faith"(Chapter 3).
 * Ys IV: The Dawn of Ys and Mask of the Sun. This theme was also remixed a few times in Ys VI: The Ark of Napishtim.
 * Some of the Mega Man games have this, eg 5, 8, and 9.
 * And let's not forget Mega Man 10 with Silent Rain and Abandoned Memory.
 * Most of Twisted Metal's songs use variations of "Asphalt Assault"'s guitar riff. The Title Theme Tune, oddly, does not.
 * Time Crisis.
 * The Halo series:
 * Halo: Combat Evolved: The classic series theme, Enough Dead Heroes, Shadows, etc.
 * Halo 2: The Last Spartan, the Arbiter's Theme, High Charity, and the Delta Halo theme
 * Halo 3: Finish The Fight, Farthest Outpost, and numerous motifs from the previous games
 * Halo: Reach: Lone Wolf, Remember Reach, Ghosts and Glass, etc.
 * Fortunately, all the aforementioned themes are already pretty kickass already, and the variations tend to be quite different. Also, there's actually a huge wealth of original, non-copied music in each Halo soundtrack.
 * The entire soundtrack of Golden Eye 1997 is, fittingly, variations on the James Bond theme song.
 * The game of The World Is Not Enough does not have the classic Bond theme, but it does use variations of the eponymous theme from the film.
 * The GoldenEye film didn't have the Bond theme either; conversely its theme tune was (mostly) absent from the game.
 * Perfect Dark and Perfect Dark Zero also do this to some degree.
 * Many of the tunes in Syphon Filter: The Omega Strain, such as Logan's Theme, Carthage Mall (Action), St Cetteo Square, Belaya Vezha (Action), Taherir Palace, etc. are variations of the main title theme. The PS 1 installments also have their own recurring themes.
 * In Resident Evil 2, William Birkin/G's theme appears throughout the soundtrack, including the Street, Courtyard, Front Hall, Marshalling Yard, and Extreme Battle themes.
 * Nemesis's theme in Resident Evil 3: Nemesis. "Feel The Tense" when he's nearby but offscreen, "Nemesis Theme" when you fight his first form, "Unstoppable Nemesis" during the battle where he infects you with The Virus, "Nemesis Again" for his second mutation, "Nemesis Doesn't Give Up" when you fight him in the Treatment Room, and "Nemesis's Final Metamorphosis" for the Final Boss battle.
 * Many of the Tomb Raider games do this, eg the original trilogy uses many variations of the title theme (generally considered Lara's theme).
 * Most of the Medal of Honor games. The first few games mainly use variations of the title themes, the player characters' themes (e.g. Patterson's themes in the first game and ''Frontline), and the recurring Nazi theme, along with motifs such as "Locating Enemy Positions", "The U-Boat", "Panzer Attack", "Border Town", "Clipping Their Wings", "Sturmgeist", etc.
 * No More Heroes has it's main theme, a jazzy little 1:30-ish tune. Which is remixed into a rock version, a techno version, a trance version...which isn't to say the entire OST is comprised of them, but you'd better get ready to get used to it.
 * Midna's theme / the Hyrule Field theme in The Legend of Zelda: Twilight Princess is the basis for much of the soundtrack. A Link to The Past also does this somewhat with Zelda's theme, including the opening and ending themes.
 * The Journeyman Project has a new-age version of its main theme during the opening cutscenes, a muzak version in the Caldoria Heights apartments, a synth-orchestral version in the TSA, and a rock version for the Ending Theme.
 * Though it has a very extensive soundtrack otherwise, almost all of the original music in Super Smash Bros Brawl is based on the opening theme, by Nobou Uematsu.
 * The Star Wars: Rogue Squadron series, in addition to using many songs from the movies, uses variations of the series' own theme.
 * Conkers Bad Fur Day remixes the Windy area theme to make the Barn Boys, Bats' Tower and Uga Buga theme.
 * Banjo-Kazooie: Nuts & Bolts' soundtrack is based on orchestral remixes of old melodies from the first two games.
 * NieR's soundtrack is predominated by "Ashes of Dreams", which has several variations on its own... and then shows up in "Dispossession" and "Yonah", both of which have their own variations. No complaints from the fandom, though.
 * In addition, several other themes return in variant forms:  gets a dramatic Dark Reprise for the boss fight with , "Kaine Salvation" returns as the lively "Kaine Escape", melancholy "Emil Sacrifice" becomes the Triumphant Reprise "Emil Karma", and "Shadowlord" shows up several times.
 * The SNES version of SimCity used variations of a main theme for each stage of your city's development.
 * The PlayStation 2 Spy Hunter games use various rock and techno remixes of the Peter Gunn theme.
 * Modern Warfare 2's soundtrack is composed of a number of tracks based on the original six-note riff heard at the end of the introduction.
 * Like other games scored by Kazumi Totaka, Wave Race 64 uses this for nearly every track.
 * Descent 3's Theremin-based title theme is remixed throughout the game, although there are also a few original music tracks.
 * Each landmass in Hyperdimension Neptunia has its theme remixed once for its dungeons and again for the battles within said dungeons. The title theme has also been remixed and used in cutscenes at least a few times.

Western Animation

 * Inspector Gadget. Gadget's theme in particular is arranged in a different way in every episode, and is a Recurring Riff in most of the other tracks as well. Nearly every track on the show's soundtrack contains at least a small bit of it. The other unique songs were often rearranged as well; Penny's theme had at least four different arrangements, probably more (though only one ended up on the soundtrack).
 * This is actually a common occurence in many TV shows, especially animated ones. Even anime does this every so often.
 * The Simpsons is deservedly famous for the many variations of its main theme, sometimes in the style of those of well-known movies or TV series or rescored to be played by e. g. a Jazz quartet, rock bands or renaissance musicians.
 * The Voltron series used the theme motif (A C# D E) quite a bit in its soundtrack.