Musical Trigger

A specific musical cue—not in the BGM, but in actual music being played by someone or something In-Universe—serves as the trigger for some event. The more dramatic the event, the more likely that the trigger occurs at the end of the song, in which case the song functions as a melodic (and quantum-mechanical-anomaly-proof) form of the Dramatic Countdown Clock.

Diabolical Masterminds are particularly prone to incorporating music into their master plan. Maybe manipulating a public performance to their own ends is their way of showing off their Chessmaster cred, or maybe they're just Wicked Cultured. The downside of this, of course, is that any hero can bring the plan crashing down simply by disrupting the song.

The Xylophone Gag is a very specific subtrope of this. If the song itself is harmful, it's a Brown Note. If the song is part of a magical spell, it's Magic Music. If the song opens a door, it's an example of Songs in the Key of Lock.

Examples of Musical Trigger include:

Anime

In Read or Die, a clone of Beethoven is a big part of the I-Jin's plans; his Suicide Symphony causes anyone who listens to it to commit suicide.
Elfen Lied has a music box playing the theme song "Lillium" that causes Nyu to shift to her virulently murderous side Lucy.
Sorta used in Tantei Gakuen Q, where the Banquet of Evil melody is used to bring a violinist insane with guilt and fear - years after he and other violin players conspired to get their biggest rival to inherit their teacher's prized violin out, which results in her being mutilated and commiting suicide afterwards.

Film

The Man Who Knew Too Much: One specific cymbal clash in the symphony was the assassin's cue to fire—that was the loudest moment of the symphony, so it would have masked the sound of the gunshot.
The Great Mouse Detective: The trigger of Ratigan's Death Trap for Basil was tied to a phonograph tonearm, so it would set off the trap when it reached the end of the record.
Zoolander: The song "Relax" causes Derek's brainwashing to kick in, transforming him into a mindless assassin.
Get Smart: The final notes of Beethoven's 9th Symphony are the signal for the detonator.
Eagle Eye: The pentultimate trumpet note of "The Star-Spangled Banner" is the signal for the detonator.
- Also an example of Did Not Do the Research, as the same note is played much earlier... "And the rocket's red glare."
  - Might be a lack of research, but don't forget, the bad guys specifically arranged for that last note to be extra-long.
Serenity: River Tam has been programmed to go on a killing spree when a subliminal signal, hidden in a Fruity-Oaty Bar commercial, is played.
In The Living Daylights, James Bond has a keyring that emits a stunning gas when he whistles Rule Britannia and explodes when he gives a wolf whistle.
In The Three Stooges movie Punch Drunks, Curly goes berserk whenever he hears "Pop Goes the Weasel".
Likewise, in the Laurel and Hardy movie Saps at Sea, Oliver Hardy goes berserk whenever he hears horns.

Fan Fiction

In Olga Chigirinskaya's novel Beyond the Dawn, a particular song performed by a particular minstrel was the key to undoing the protagonist's Memory Gambit.

Live Action TV

In Family Matters, one episode had Steve Urkel invent gunpowder that was set off by the finale of "Roll Out the Barrel." When it was mistaken for pepper and used to cook dinner for a party, hilarity ensued.
Buffy the Vampire Slayer: Spike's trigger in season 7 is an old English folk song, "Early One Morning."
In The West Wing, Josh is counseled by a psychologist due to his recent unpredictable behavior, and it turns out the trigger was the Christmas music in the lobby. In his mind, music represented the ambulance siren and made him relive the trauma of being shot months before.
An episode of Get Smart featured a cannon that would fire at the climax of Tchaikovsky's 1812 overture.
Battlestar Galactica - the jump co-ordinates needed in the series finale were derived from Starbuck playing the main theme from All Along The Watchtower. While being nuked near a black hole, no less.

Video Games

The Legend of Zelda is quite fond of these. The most famous example is the Ocarina of Time from Ocarina of Time and Majora's Mask; various melodies on the Ocarina can teleport The Hero to set locations, change night into day, summon a rainstorm, reverse time, and so on. The keystone melody in the game - Zelda's Lullaby - is basically a generic activator for a wide variety of effects.
- Other musical triggers in the franchise include melodies from The Wind Waker's titular conductor's baton and the Harp of Ages from Oracle of Ages, grass whistles and Wolf Link's howls in Twilight Princess, and the Ballad of the Wind Fish in Link's Awakening. Several games, such as A Link to the Past and The Minish Cap, also feature a flute or similar instrument used to summon a bird to fly Link to set checkpoints, while the Oracle saga games featured flutes to summon Link's mounts.
- Brawl in the Family took the Lullaby's relationship with the Triforce to its semi-logical conclusion.
The Forgotten Superweapon Vegnagun in Final Fantasy X-2 is operated by playing an elaborate tune. Good thing the only person who remembers it is dead... Wait, he's come back as an Unsent? Oh Crap.
Left 4 Dead 2: Get Jonathan Coulton's Re: Your Brains song to play on the jukebox and the chorus will result in a Horde being summoned.
This is how you get the best ending of Chrono Cross. In the fight against the Time Devourer, each element color plays a corresponding note when used. You free Schala by using them in the right order to play a specific tune.
Shivers. By playing the siren's song on the organ in the "Mysteries of the Deep" room, a door opens up to reveal the way to the "Subterranean World" maze.

Web Comics

In Girl Genius, Agatha uses an organ to control her "battle circus".

Western Animation

South Park: Don't forget the late Middle Park Cowboys' mascot, to whom was attached a bomb rigged to explode when John Stamos's brother Richard hit the high F in Lovin' You. Of course, everybody knows Richard Stamos can never hit the high F...
- Until he finally does in the last few seconds of the episode.
A Pinky and The Brain cartoon involved Brain encoding a subliminal message in a typical country song.
Chaotic uses Mugic, musical magic.
In an episode of Tale Spin, the gate to a lost Advanced Ancient Acropolis is opened by singing a perfect B note, which the first goon sadly cannot.

Klang: She said B, not B flat!

Thunderbirds offers the cham-cham, a coded musical instruction to air pirates signalling the time to attack and directing them to their target. When this is discovered, Lady Penelope sings an altered version of the song, redirecting the hapless pirates' flight-path right over a military airbase.
In the Batman: The Animated Series episode "If You're So Smart, Why Aren't You Rich?", a door has three possible keys, labelled "A, C, and D". The D key causes two blades to fly at Batman and Robin. When Batman tries to use the A key, Robin stops him, saying there will be three blades--the Key of A and the Key of D have three and two sharps, respectively. The correct answer is the Key of C, which has no sharps. They get through the puzzle.
A Looney Tunes cartoon has Daffy Duck trying to sabotage a Bugs Bunny concert by rigging the piano with explosives. However Bugs keeps misplaying the tune and skipping the trigger piano key. Finally Daffy, frustrated, kicks Bugs out of the piano to show him how to play the tune correctly- and of course blows up. (As Bugs probably intended.)