Variable Mix

The music for a non-interactive story, like a film or TV show, is expected to change and adapt to the action taking place. However, games have a limited ability to do this. It's not practical to have John Williams watch the player's actions and spur up the London Symphony Orchestra every time Solid Snake gets seen by a passing guard.

A hard cut to a different piece is one possible solution for a game, and it's not so jarring when disguised by a sound effect. But some titles make better use of their technology: in a Variable Mix, the background music changes subtly and smoothly depending on what is going on in the game. The running background music could have, for example, parallel parts that fade in and out with the rising and falling action level, seamlessly transitioning from a bare-bone ambient haunting theme to a hard-rocking drum-backed metal anthem during combat, and back again after the last enemy has fallen. A clever bit of composing that has a very subtle but real influence on player immersion. Variable mix is almost always made possible by MIDI (musical instrument digital interface).

Often overlaps with Musical Spoiler for more Genre Savvy players who know what particular variations to listen for. Can sometimes enter a variant of Mickey Mousing known as Musical Gameplay. See also Theme Music Power-Up, which this often turns into if it happens during a boss battle. Also, see Songs in The Key of Panic for the other end of the scale.

Action Adventure

 * Luigis Mansion repeats the main theme of the game when you're in the Mansion, but it has variations for hallways, dark rooms, and outside. All have Luigi humming over the theme, except in lit rooms, where he whistles the theme. The tempo also decreases if Luigi's health worsens.
 * The poor guy's humming sounds more and more scared and shaky as his health lowers, too!
 * The Legend of Zelda: Link's Awakening does this (and on the original Gameboy, too!) with its central Ballad of the Windfish - once the first two of eight instruments are collected, the Ocarina becomes obtainable, and the song can be played in front of the big egg... but it will sound rather empty. As more instruments are collected, the process can be repeated, until having all eight summons the full song. And the 3D Zelda games would make a habit of this in the overworld:
 * Ocarina of Time had a variation in that the main overworld theme was made up of separate segments, which would play in a mostly random order, but it would pick slow segments when Link was standing still, trumpet fanfares when he was moving, and dramatic music when there were enemies present.
 * The Ominous Pipe Organ music in Ganon's Tower changes in melody as you go up the tower.
 * The Legend of Zelda Majoras Mask saw the Clock Town theme change depending on what day it is (and, therefore, how close the end of the world is). The first day is upbeat and cheerful, the second day is faster as people begin to realize that the moon is getting bigger. On the third (and final) day, the music is frantic, with a very ominous backing-track.
 * Wind Waker saved the trumpets in the overworld melody for when Link was traveling at full speed. In battles, dramatic flourishes are added as you hit enemies, and in boss fights, different variations of the boss's theme are used (e.g. the Helmaroc King's music gets a bass line once you remove his crown.)
 * The Mini-Boss theme gets a special mention, as it has versions for- sword sheathed (Soft), sword drawn (Hard), near enemy (drums are added), low health (fast), landed attacks (the added chords as per usual), landed ENEMY attacks (pitch lowers while the song speeds up), and even a specific horn roll plays while using the spin attack. All of these layers play independently, immersing the player into the action better than any other song in the game, but unfortunately, these battles typically only last a few minutes tops.
 * Twilight Princess has two Boss Battle themes, the bleak theme when the boss has an advantage, and the upbeat theme once Link finally gets close to its Achilles Heel. Furthermore, in Hyrule Field, if you get on your horse, trumpets and other "epic" instruments are added to the music's arrangement, as well as additional melody phrases. If you stand still for a while, the music will slow down into a soft backing track; running off again will cause it to continue. As the sun sets, the music slows down, and segues into the nighttime music. In the morning, when the sun rises, "morning" music plays and leads back into the theme. Finally, once you reach, the music starts out as the  theme, but slowly turns into   theme, one instrument at a time, the closer you get to the end, until you reach the end, where the former's theme is completely drowned out by the latter's.
 * Also, in the Sacred Grove, Skull Kid's trumpet is added to the music if you're near him, and there's a variation of the music if those puppet things of his are near you.
 * Also used awesomely in the final boss battle against . When you go into a chance, which is when both you and him have swords locked and are trying to push the other down, the music will change depending on which side is winning, sounding ominous when   is winning, and playing the game's main theme when you are winning.
 * Koji Kondo says that the reason he didn't use orchestrated music for Twilight Princess was because he apparently had trouble implementing Variable Mix with non-MIDI music with Game Cube hardware. If Super Mario Galaxy is any indicator, however, such troubles will likely be a thing of the past.
 * Spirit Tracks has different music tracks that fade in and out as you change the train speed.
 * The chugging of the train's engine in its top gear syncs perfectly with the standard overworld music, making it into a kind of percussion line.
 * The music when climbing the Tower of Spirits becomes more epic the higher up you go.
 * When fighting a Rocktite, the music becomes more menacing the closer it is to your train. When fighting Cragma, the music gains instruments when riding the cart... Spirit Tracks uses this a lot.
 * Skyward Sword just might contain the largest amount of variable mixing in a video game to date, as most areas, dungeons, battles and cutscenes take advantage of the technique. A notable early area where this can be seen is the Bazaar, where each shopkeeper has their own little Leitmotif that's a variation on the main one set to play when you approach their stall.
 * In Shadow of the Colossus, the music transitions smoothly to a more upbeat theme once you have discovered how to get onto the colossus.
 * In NiGHTS Into Dreams, the variable mix used was determined by the happiness or lack thereof of the Nightopians in the current level.
 * Brave Fencer Musashi did this with the Alucaneet Palace theme only. The subdued BGM you initially hear is performed by a rather spare number of instruments, but for each musician you rescue in the field, an instrument is added to the orchestra until the palace theme sounds downright energetic.
 * Both Mystical Ninja Starring Goemon and Goemon's Great Adventure utilize this effect in the castle areas, with the music changing to reflect the player's progression.
 * Mushroom Men has various environmental pieces that make sound effects in time with the background music, such as sparking electrical contraptions and dripping faucets. The "Jackalope" boss also features music that gets more and more intense the more damage has been dealt to the boss.
 * In Okami, going into important and/or busier areas of some maps would add an extra layer of music.
 * Most "overworld" musics have two versions. Which one is playing depends on plot point and the time of day, most of the time.
 * Akuji The Heartless for the PS 1 did this pretty well. Each stage featured a unique musical theme that not only got more hectic when the player entered combat, but also added new layers, changed tempo, or became more ambient when set pieces were triggered.

Adventure Game

 * A large number of classic Lucas Arts games, beginning with Monkey Island 2, used a system called iMuse to provide interactive music, one of the first such examples of this. The most famous example is probably MI2's Woodtick, where the music has dozens of transitions meaning that each piece of music comes to a natural end, wherever it is in the track, before moving on to the next piece. The result is mindblowing, and it's the kind of thing which is only possible with MIDI.
 * The Special Edition tried to recreate the effect with live instruments. They did a pretty good job, but the effects were much simplified in many cases.
 * X-Wing and TIE Fighter used this too. Needless to say, when used in conjunction with elements of the Star Wars score, plus new music that blended with it, the effect was extremely enjoyable and atmospheric.
 * As does the Rogue Squadron series.
 * iMuse was used well in The Curse of Monkey Island. Depending on who you were talking to, the background music would segue smoothly into a remixed version; the Barbery Coast theme would gain an accompaniment of accordion when talking to Captain Rene Rottingham, and when on the Sea Cucumber, steel guitar, steel drum, and atmospheric seagulls would be added to a more upbeat version of the Sea Cucumber theme when Mr. Fossie came to harass you.
 * Starship Titanic features many music themes which blend into each other, and you soon notice that the quality and style of the music depend on how posh your surroundings are, and whether or not you're in an public or maintainance area.
 * Older Than They Think: In Space Quest IV, the Galaxy Galleria theme is normally elevator/lobby music, but when you enter the Skate-O-Rama, it adds drums. It also mixes in the various shop music themes as you pass by.

Card Battle Game

 * Most recent Yu-Gi-Oh video games use three different songs during each duel: one for when the Life Points are close to even, one for when you have a big lead, and one for when your opponent has a big lead. The music simply fades out and fades in once the threshold is reached, however, without any fancy attempt to blend the songs together.

Driving Game

 * Diddy Kong Racing as well; not only on the racer selection screen (each racer had a unique instrument playing a variation of the same melody over the same accompaniment), but the track Boulder Canyon switched between a rock organ, a flute, and a trombone depending on where you were in the lap. The only problem was that if you listened to these tunes on the in-game Sound Test, it would play every instrument and melody at once.
 * Also occurs in the Hub Level.
 * In Mario Kart Wii, some tracks have added sound effects inside of caves. A distortion effect is added whenever the player is shrunk or flattened due to item or track effects. Finally, when starting a game, the music has more layers added to it the farther you get through the selection process of the type of game you'll play, the difficulty, the racer you want, the car you'll use, and what sort of drifting you'll be playing with.
 * Mario Kart 7 adds extra percussion (bass/snare "dance" beat or a hi-hat, for example) to the BGM if the player is in the lead.
 * Burnout 2 normally has a repetitive drum beat and electro soundtrack with a secondary backing track that is muted unless the boost is triggered, which pulls it back into the mix and gives the music more of a surfer-tune.
 * The original game handled things differently. The foreground music would play during unboosted periods then would fade into a sharp violin track when the boost was triggered.
 * The levels in Twisted Metal 1, 2, and Black can have up to three dynamic music variations: the ambient music(eg "Cyburb Hunt" in Cyburbia), the battle music(eg "Cyburb Slide"), and the "final opponent" music (Black, and maybe Head On).
 * "Cyburb Hunt" sounds like it was supposed to be a dynamic variation of "Asphalt Assault", but River Park uses "Stalk n Roll" for its ambience and Cyburbia uses "Cyburb Slide" for its battle theme instead. Programming error?
 * Need for Speed III: Hot Pursuit did this in spades, and did it VERY well. There were a limited number of courses in the game (the total being nine) and all courses except shared music with another course. Still, each course had two music tracks: a rock and a techno track, meaning ten pieces of music (outside the menu themes). However, each of these tracks was composed of three second segments that when placed together usually totaled well over 10 minutes of music (and in fact, many of these segments never made it into the official soundtrack). The music flowed absolutely seamlessly, which is a good thing, as the music would change depending on three main factors: Your location on the course (where you were), your rank (in the race) and your speed. There was even a special bit that played when you crashed, as well as one when being chased. Given the fluidity and constant changing circumstances of a vehicular racing game, you can imagine the challenge this must have been for the composer.
 * This also applied to some tracks in NFS II but didn't have the chase segments. Examples could include Gore (for the track in the U.S), Corroboree (for the track in the Land Down Under), Sanquoa (for the track in Canada), and some others while some tracks didn't share this variable mix, such as Halling Ass and Headless Horse for Norway's Proving Grounds (since it was a simple oval track so a variable mix would've been quite unnecessary).
 * Need for Speed: Most Wanted does this as well, albeit only during the Cop Chases. While it can get a little repetitious sometimes, it's still (arguably) the best music in the game.

Fighting Game

 * In Super Smash Bros Brawl, the main tracks of the stages Summit and Yoshi's Island constantly change in tone and tempo to match the current state of stage (Though the Summit music is a single audio track that's merely timed to match up with the stage's various events; if you're playing in Slow Brawl, for instance, you'll hear the audio desynch from the onscreen events).
 * In Def Jam - Fight for New York used a variant that revolved around the Blazin' Moves. Most stages in the game have instrumentals of various hip hop songs as their BGM, which cuts out to one of a few slightly more intense, shorter loops whenever someone activates their ability to use a Blazin' move. If the successfully pull the move off, Ominous Latin Chanting or some other brief, suspenseful leitmotif plays in the background while the attack is being performed, and, if the Blazin' move is used when the opponent's health isn't in the danger zone and is able to continue fighting afterward, the normal music resumes and for about ten seconds afterward the lyrics of the song play before cutting back into a looping instrumental again. A lesser example occurs whenever you use a submission hold, with disk scratching and various generic sound effects thrown in.
 * In Street Fighter X Tekken, the music increases in intensity for each round of a match. In addition, a flange effect coats the music when the player's current character's HP is below 25%.

First-Person Shooter

 * Tron 2.0 had a rather smooth transition between normal wandering-around music and when combat began.
 * Ditto for Deus Ex.
 * And Unreal.
 * And Serious Sam.
 * The Halo games, particularly the second and third, use what the composer calls "quantum music". A piece may have an intro, followed by a looping middle section that may have different variations depending on the player's actions, and an outtro, again triggered by the gameplay. The music fades out after a certain time if no dynamic triggers are activated. "Glue" musics consisting of short loops (typically drone ambient) are used to transition between larger pieces.
 * Return to Castle Wolfenstein: The music is mostly silent, with the occasional drums, but if an alarm goes off or something, it will segue to a Battle Theme Music piece, ending once the alert is stopped. And during timed missions, the music intensifies periodically as the clock runs down.
 * Far Cry 2's battle music had a bad habit of fading in whenever the player fired a shot, regardless of whether or not there were any enemies around.
 * In Golden Eye 1997 and Perfect Dark, each stage has a "danger" variation of its music.
 * No One Lives Forever, an FPS taking place in the Spy Sixties, features both sneaking and flat-out shooting elements, and the music rises and falls appropriately if you blow your cover.
 * FEAR: The music starts to build in suspense just before a major event or battle happens, and segues out once the battle is over. For example, the courtyard in Interval 3 where the helicopter drops off several Replicas. And Scare Chords are often set to the action, e.g. when the gun turrets activate.
 * Descent 3's level musics seamlessly change as you progress through a level, and also change with the intensity of the action.
 * Some levels in the Medal of Honor series have this, such as "Eye of the Storm", "Rough Landing", "The Golden Lion", "Operation Repunzel", "Yard by Yard" and "On Track" in Frontline, when you reach certain points in a level, an alarm is set off, or the action otherwise heats up. Rough Landing in particular is awesome; it starts with a solo boy soprano and a few instruments, and later in the level, it builds up to the full choir and orchestra. Also, in the train mission, the music starts to pick up speed when you enter the station and approach the train, then goes to its undoubtedly epic full-speed Ominous Chanting climax during Riding Out The Storm.
 * System Shock 2 was another early example: each level had a number of short variations on a theme that would chain together depending on the circumstances.
 * Fallout 3 and Fallout New Vegas have this, with non-radio ingame music changing depending on various things, such as the player arriving at certain areas or being discovered by a passing enemy.

Hack and Slash

 * Chaos Legion has a normal level music, and adds Ominous Latin Chanting when your have a legion out.

Mecha Game

 * Zone of the Enders spliced in different loops to the mostly trance in-game music depending on whether Jehuty was shielding, locked onto an enemy, trying to escape, etc.
 * Zone of the Enders: The 2nd Runner, for the second stage, had an ominous choir background music with industrial drums fading in and out with the action. A rather memorable scene occurs at the end of the stage when the protagonist reaches the end, gazing out the battleship windows out into space as the drums go out entirely. Have a listen - this version has the drums almost the whole way through.

Miscellaneous Games / Multiple Genres

 * Action 52 has this unintentionally in level 3 in Lollipops. While belonging to glitch chiptune subgenre, it changes with every action like walking, jumping and attacking.
 * Rescue: The Embassy Mission for the NES had this during the sniper positioning sequence. If your sniper was hiding, the music became subdued; if your sniper was running, more instruments come in.

DJHeros

 * Phantasy Star Online has two themes for each area, one for exploration and one for battle. There are around five transition points in each track - once a battle begins or ends the game will wait for the next of these points and cue the transition to the other theme.
 * In World of Warcraft, the PVP only zone Wintergrasp uses a number of different songs dependent on what's happening in the zone at the time: three or four calm tunes for when battles have died down, and an equal number of high energy war songs when players are vying for the fortress.

Party Game

 * An unusual example comes from Wario Ware: Smooth Moves on Dribble and Spitz's stage. The vocal song "Tomorrow Hill" plays throughout the level. When you mess up, the song distorts. You might think it's just an effect placed over the original song, but it's actually a Variable Mix- the distorted parts of the song are actually sung differently, sometimes even with nonsensical words in place of the normal ones ("Already said my goodbyes" becomes "Already ate my french fry", for example). This alternate version of the song, "Falling Off Tomorrow Hill", can even be heard in the Sound Test.

Platform Game

 * Sly Cooper
 * Super Mario Bros 2: The music continues playing when the game is paused but cuts out the main instruments, leaving a sort of "drum and bass" mix.
 * Super Mario World: When the player mounts a Yoshi, a track of bongo drums is added to the music, no matter which of the several background music tracks is playing at the time. The same thing happens in Super Mario Sunshine, New Super Mario Bros Wii, and Super Mario Galaxy 2.
 * Super Mario 64 has its water theme. Example : when you are on the beach in Jolly Roger Bay, the music is simple. If you go in the water, it gains violins; and the hidden cave add drums to it.
 * Something similar happens to the themes of Hazy Maze Cave and Wet Dry World. Normally, it plays a remix of the SMB Cave Theme. However, once you're at a certain area, the theme starts adding a few creepy instruments.
 * Taken to epic levels in Super Mario Galaxy in the boss battles against Bowser whenever he becomes open to attacks. The song is normal until you get Bowser to land on the lava; when that happens and he runs around with his tail ablaze, the singing chorus comes in. When you hit him and he is spinning on his shell, a trumpet is added in with the song and singing chorus until you hit him again, then the song returns back to normal until you do it again.
 * Also in Galaxy, every time you activate a sling star a harp will be added in tempo and harmony. If you activate the bigger ones with two stars (to go to different planetoids, etc.) the harmony will be longer; if you use the single star ones for shooting you short distances, the harmony will be shorter. This occurs with every single song, even with the Rainbow Star (invincibility) and power-up songs (Fire Flower, Ice Flower, etc.)
 * Any coins that pop up from bushes, etc. will be in tempo and harmony with the song.
 * Occurs in the tilty-ball levels. The less you move, the quieter the song gets and there is only one instrument playing, but, the faster you go, the faster the tempo gets and the more instruments that come in.
 * Done in the Battlerock and Dreadnought Galaxies when the level switches from a quiet, calm section to an action filled section with cannons and Bullet Bills, with the music getting more epic or just quiet accordingly. The Beach Bowl and Sea Slide Galaxies work similarly, with the loud brass and high flute switching out for steel drums and other luau-ish instrumentation when you dive underwater.
 * same case with Buoy Base: above water, it's Symphony Orchestra, below water it's synthesiser and Organ.
 * The question-mark color-changing platform puzzles dealt out a tone every time you stepped on a separate platform. Each tone would be in tandem with whatever chord was currently being used in the melody of the stage.
 * In the Banjo-Kazooie games, the music would change slightly depending on where the player is in the level. For example, the music would always change to a harp arrangement when diving underwater. And in the first world, Mumbo's Mountain, a military drumbeat would be added to the track when the player approaches the termite hive.
 * Gruntilda's Lair alone had at least 11 different themes that would blend seamlessly from one to the next: The base theme music, the underwater music, and one for each world that would play when you approach a world's portrait or entrance, with an arrangement appropriate for that world. Cheato's theme may also count - it's not a variation on the Gruntilda's Lair theme but the music switches from the lair theme to Cheato and back when you approach/walk away from him, with one fading out at the same time the other fades in so that it feels seamless.
 * In Banjo Tooie, there was a glitch where after beating Mr. Patch, sometimes every variation of the Witchyworld theme would play at once until you exited the tent.
 * Also done in Conkers Bad Fur Day.
 * Basically, Rare (David Wise, Grant Kirkhope, Eveline Fischer, ect.) loved this trope and used it at every opportunity.
 * Happens in Donkey Kong 64, albeit not on the fly. The main hub theme has variations depending on whether or not you're near DK Island, near the Kremlings' fortress or in one of the level hubs.
 * even if your character is not in an area where the music changes in a level, if you do something that triggers an event in such an area, the music will change as the camera enters that area.
 * In Donkey Kong Country Returns, the island map has the well-known "Simian Segue" as its BGM until you zoom to any part of it, where it fades to a different rearrangement (with the same rhythm, no less) of that theme in every section, using the appropriate instruments (for example, steel drums in the "beach" map).
 * Jet Set Radio Future has horribly abrupt music changes, or no changes at all, depending on what part of the game is occurring, but this is expected as the music is coming from the players own CD player or radio most of the time.
 * The games in the Jak and Daxter series add new layers to the background music when the player mounts a vehicle or takes out a weapon.
 * Yoshis Island has a variation on this trope: with each world completed, an additional line of instrumentation is added to the map screen music, until it becomes fully orchestrated on the last world's map.
 * Paper Mario reused the tune for Lava Lava Island, and each area of the map had a different mix of the tune. In fact, lots of areas in the game were like that, but the most dynamic had to be Toad Town, where the different parts of the tune would actually fade in or out as you approached various areas.
 * Toad Town has its own theme song, and certain parts of it have their own themes, which are in the same key as and in sync with the Toad Town theme. When you are near one of these parts, you can hear the special music just a little bit, and when you are in said part, the special music is all there. The main theme can still be heard faintly, as if it were coming from outside.
 * In Yoshis Story, the music changes depending on your health. When you're down to your last health point, the music slows to a crawl. When you're invincible, it changes to an electric guitar version of the level theme.
 * Ristar has a music-themed level in which the objective was to deliver metronomes to birds found throughout the level. For each metronome returned, a portion of the level's background music was replaced with a choral melody.
 * The final boss theme isn't a layered track like Planet Sonata, but it aims for this effect in spirit. It's timed so that the slow part lasts almost exactly as long as it takes to wear Kaiser's first phase down, the accelerando takes place during his first black hole attack, and the fast, frantic part goes into full swing when he Turns Red.
 * In Tomb Raider: Anniversary, the background music in Croft Manor changes slightly depending on whether the player is in the house, the gym or outside.
 * De Blob features a rather interesting soundtrack that adds instruments as you colour buildings in different colours.
 * At the start of each level you can choose which soundtrack to use, with a variety of available instruments. Additionally, more background music will play as you paint an area- a gray zone will be quiet but a fully painted area will sound quite alive.
 * Used in Knytt Stories, as the player transitions between areas.
 * In the original Klonoa game, this trope was used two different ways in the same place: the between-level transition screens. Each time you visited it, each level you'd completed would play a section of a tune; completing every level would complete the tune, . In addition, each level had six characters to rescue, who formed members of the band that played the tune; if you didn't collect all six characters in a level, their portion of the tune would have correspondingly fewer instruments.
 * Glider PRO's music goes into a holding pattern if you stay in one room too long.
 * In the pre-Adventure Sonic the Hedgehog games, the BGM currently playing would speed up if you got the super shoes (only Sonic CD and the Saturn version of Sonic 3D Blast didn't do this — presumably technical limitations since they were playing direct from audio CD).
 * Sonic Heroes gives us Mystic Mansion with its segmented level theme; different segments play and loop depending on where in the level you are, transitioning at noticeable "checkpoints". You can hear them all in sequence in the Sound Test. The preceding level (Hang Castle) toys with this as well, albeit differently. Its two themes are interchanged virtually seamlessly as you go from normal castle to upside-down castle and back again.
 * In Sonic Generations, the music would get more intense when boosting in Modern Sonic's stages.
 * This originated in Sonic Colors, where, upon boosting, the prominent instruments in the BGM would be drowned out a bit, allowing the drum & bass to come center stage. Diving underwater with Sonic in Aquarium Park also creates subtle changes in the music.
 * Reversed in Sonic Unleashed where the drum & base would be drowned out, sometimes completely, when boosting.
 * In Donkey Kong Jungle Beat, the background music is often a simple, understated little tune. And if you play through the levels like a normal platformer, it'll stay that way. However, once you start using the combo system (doing tricks in midair and stringing moves together without touching the ground), things get interesting as you collect more and more bananas. More instruments will be added to the song. Then the percussion begins to pick up. Then a crowd yells "GO! GO! GO!" to the beat of the song. Then the crowd also yells "Wooo!" on the off-beats and claps along. Then the main tune is overlaid with some funky jazz riffs. And then the percussion starts drowning everything out... When you finally land and end your combo, the crowd breaks into a hearty cheer, and then it's back to the simple music again. Needless to say, playing well in this game gets you pumped.
 * Don't forget the boss music, which effortlessly switches from tense to triumphant, depending on whether DK is on the defensive or the offensive.
 * Kirby: Canvas Curse uses this in the final battle against Drawcia. The first part of the battle has kind of slow music while Drawcia uses basic attacks. Then it kicks into high gear when the screen Turns Red and she begins using her signature paint powers.
 * Epic Mickey has three different versions for each different land's theme (for the purpose of this article, we'll use the Mean Street theme as the only model). These are Paint, Neutral, and Thinner. What version plays depends on how many Guardians you attract and what kind they are. If you have no Guardians, the Neutral version plays. If you have one or two Tints, the Paint version is layered over the Neutral version. If you have all three Tints, the Paint version is the only version that plays. If you have one or two Turps, the Thinner version is layered over the Neutral version. If you have all three Turps, the Thinner version is the only version that plays.
 * In Oddworld: Abe's Oddysee and Abe's Exoddus, the score shifted from tense-hallway-sneaky to dramatic-drumbeats to kickass-and-chew-bubblegum as Abe possessed a slig. And whenever Abe is spotted by guards or wildlife(Scrabs, mostly), the usually calm background sounds shift into a fast-paced chase music.
 * In Metroid Zero Mission, you're caught without your power suit and trying to stealth your way through the space pirates' mothership. Assuming you're not a video game god and failed to complete the entire section without getting spotted a single time, the music in the area changes from a softer version (when the pirates aren't chasing you) to a more urgent, louder version (when the pirates Are chasing you).
 * In the GBA version of Donkey Kong Country 3, which has a new soundtrack, the riverside levels have this, with the ambient+bongo track when out of the river, and an aquatic theme for when you're in the river. Of course, with most of the river levels eventually hurting you if you end up in the river...

Puzzle Game

 * Worms Blast used this as the blocks got closer and closer to you.
 * Cipher Prime games Auditorium and Fractal do this. In the former, each level starts in silence. Every "bucket" you fill with the correct type of flowing particle adds a layer to the music. In the latter, creating more blooms adds these layers. Each level starts with the stereotypical "howling wasteland" sound.
 * In some levels with Portal 2, the background music gets instruments or changes when various puzzle elements are used. For an example, music becomes muffled inside excrusion funnel or gets a line of instruments when running on propulsion gel.
 * Actually, the excursion funnels always play the same tune (though I think the natural music is barely audible in the background), and the gels work differently -- the Blue gel always plays the same weird little tune when you bounce on it, and the repulsion gel applies the same "instruments" that any High Velocity situation would, Gel or no Gel. Aerial Faith Plates always add the same instruments/tune to whatever music is being played too, no matter which Plate in Which Level it is. Laser receptacles do the same; this is most notable in the room with the 3 near the door that you have to light up simultaneously. All of the above (almost) always add the same set of instruments to whatever is playing in the background. In fact, these are mere sound effects that are attached to the puzzle elements themselves. To test this, simply find an aerial faith plate and drop a cube on it. Note how the sound effect plays. Next, throw a portal and drop the cube on the faith plate and back up, note how the sound effect is quieter. They are simply scripted sound effects that sound like music that meshes well with music played in those levels.
 * Examining the game's sound files reveals a mix of both overlaid effects tracks (level-specific to fit the timing of the theme for that level), and also some variations on themes for specific levels which will use a mechanic-specific effect as a segue.
 * Adventures of Lolo (for the Game Boy) had an unusual example in which the BGM shifted between four different songs depending on which direction Lolo was facing. The music also changed to a much slower song while Lolo was on flowers. It's only on some levels, and only in the European version.
 * Rotohex of the Art Style series adds to or changes the music once a new color is available.
 * Plants vs. Zombies has two mixes of each song used for a level: a calmer version used when there are few zombies on the level, and a more urgently orchestrated one when things are starting to get out of hand.
 * Meteos Wars had the music changing as the screen got filled up with blocks. Each planet in the original Meteos had 3 different soundtracks that would play as the screen filled, but the transition was noticeable as the soundtrack restarted each time. Each planet in Meteos Wars had up to 5 or 6, and the transitions were seamless.

Real Time Strategy

 * In Age of Mythology, the normal game music would change to battle music during a large battle, often triggered by an attack on a Town Center, then afterward it would often change to the original music or a "mellow mix" (their own words).
 * The "mellow mix" would also trigger if you lost a significant chunk of your population (about 50% or more) in battle, basically the game telling you you're losing.
 * Also, there's special music that plays whenever Meteor Storm is used.
 * The Westwood/EA RTS games introduced this feature with their SAGE engine-based RTS Games (Emperor and newer). It is especially notable in Red Alert 3, which has tracks that change based on varying levels of intensity of the fighting. This includes substantial and instant music changes for such things as suffering a super weapon strike, where it goes from intense fighting music to very hard, "You're SO about to die!" track.
 * Supreme Commander featured this to break up its repeating, low intensity track
 * Take Up to Eleven in the Pikmin series, which would subtly change the music depending on what time of day it was, whether or not you were in combat, carrying items back to the ship, or just walking around, and - in the sequel - how deep into a dungeon you were.
 * The music for the Final Boss of Pikmin 2 may be the most complex example on this list. The boss has four weapons that must be removed, and every time you take a weapon off, the music will gain some new element. That means five different versions of the main battle theme. Also, the music will briefly diverge to a different theme whenever a weapon is used, and every weapon has its own theme. In total, there's nine themes.
 * The same game also featured different background music depending on which captain you were currently in control of, with Louie's sounding more silly. When you cleared a level of its treasures, the Pikmin following you would also occasionally sing along with the music.
 * Demigod plays more frantic music if there is a large-scale battle going on nearby and you are on low health.
 * Dune II, being MIDI-based, is not a pure example of this trope, as it is all distinct background music tracks; however, each of the tracks in the game is less than 3 minutes long and fade out, and all of the tracks segue into one another, allowing them to fade in and out smoothly whenever the tone of combat changes.
 * Total Annihilation used redbook audio, and made an attempt at this. The first attempt wasn't that good, since it always took the first "battle" music track but this was fixed in a patch. Even after the patch, the transition was sometimes jarring, and was always associated with a 1/2 second delay because of the CD drive switching tracks.
 * In Z, the music gets more intense as the situation gets more intense.

Rhythm Game

 * In Parappa the Rapper and its spiritual sequel Um Jammer Lammy, parts of the backing track drop in and out depending on how well you're doing. When performing well, the track is as it's meant to be heard. When performing badly, the midrange might drop out, the bass remaining only barely; in a few stages the melody changes into a minor key.
 * Turning on all tracks of a Variable Mix song is the goal of the game in early Harmonix games Frequency and Amplitude.
 * Harmonix went on to start Guitar Hero series, which has the players "playing" famous rock songs by following on-screen notes, will have the guitar track of the song replaced with an out of tune mess if the player misses a note or plays the wrong note. On the other side of the spectrum, if you do well enough to activate "Star Power", the guitar track becomes a bit more "pronounced", and the audience will start clapping to the beat of the song.
 * After the publisher turned GH development over to Neversoft, Harmonix took it a step further in Rock Band: do well enough for long enough, and the crowd will sing along.
 * ''LocoRocos lets you directly control the mixing of the two tracks. Unless you are playing on easy that is.
 * As the Patapons of the game of the same name become more excited, more drum and instrument tracks drop into the pulsing background beat and they begin to sing more emphatically. When they reach Fever mode, all the instruments come in, and the Patapons begin to chant in harmony. When the spirit is onscreen, even more additional vocals are present.
 * In Loco Roco, the LocoRocos sing along with the backing track. If they're split into their components, the components start to sing in harmony, even dividing up lead and backing vocals between themselves. The more LocoRoco the player has obtained, the thicker the choir - and as soon as they're grouped back together, the amalgam sings a single-voiced melody.
 * And in LocoRocos 2, once more MuiMuis are obtained, the background music improves from just wind-instrument playing to having the friendly LocoRocos sing. Songs from the first game had similar changes, but they were more subtle and only applied to songs which BGMs were backup singers.
 * Several songs in Beatmania IIDX actually sound different on different difficulties. Usually these are slight rearrangements, but sometimes they're entirely new remixes. Here's a pretty comprehensive list -- remember that "Light7" means Normal (and was eventually renamed as such) and "Another" means Nintendo Hard.
 * There's no attempt to blend the different songs; what you pick is what you get. However, three songs ("Scripted Connection", "Anthem Landing" and "Shade", all by the same artist) sound different on each difficulty because they're different parts of the same long song - Normal ends where Hyper (hard) begins, and Hyper ends where Another begins. The home version of the version that debuted "Scripted Connection" had an Easter Egg allowing the player to play the entire song as one piece.
 * And of course, since you're playing these songs, your skill affects how closely you come to actually recreating the song. Do badly, and notes drop - because you never played them. Conversely, long blank sections in a column can be filled in with freestyle riffs.
 * In Space Channel 5, you can at various points add a guitarist, drummer or saxophone player to Ulala's retinue, adding the appropriate sting to the background music.

Role Playing Game

 * Skies of Arcadia uses two different applications of this trope. The overworld travel music acquires distinctive background instrumentation when the characters' ship flies over or near one of the several continents--tribal drums in the vicinity of the South America-derived continent, chimes in the area of the ice continent, etc. Additionally, certain boss battles can be accompanied by normal, triumphant, or desperate sounding themes depending on how the main characters are faring in the fight.
 * The Elder Scrolls IV: Oblivion has a soft fade-out transition to various musical themes whenever appropriate.
 * Pokémon FireRed and LeafGreen did this to make an otherwise dull tutorial sequence exciting. Presented with a dry sequence of blue windows explaining what buttons to press to do what, an extra layer was added to the music each screen pressed through - drums, bass, harmony, counter-melody - giving a great sense of build-up. On the final screen the melody finally kicks in, and you feel fully prepared for adventure!
 * Pokémon Black and White Versions have this happening a LOT with the various overworld themes. In nearly every town there is an NPC that will add an additional track to the music (i.e., playing a piano or singing) and their volume is set by the player's proximity to them. Additionally, the music playing in the various routes change very subtly according to the in-game Season; you may hear flutes and strings playing a melody in Spring, whilst in Winter the sounds of bells and windchimes will play the melody instead. Additional tracks are also sometimes added depending on what you're doing at the time -- several routes have a percussion layer that only plays whilst you're moving and stops when you do.
 * The music in some cavern areas also decreases in pitch the deeper underground you go, similar to the Dragon Quest and Tales of Phantasia examples below.
 * Used for great effect at the badge check gates to Victory Road; each gate you go through adds another instrument to the music, until finally you've made it to Victory Road with the full music track playing.
 * The Genesis RPG Phantasy Star III also tried to do this for it's battle theme, many years before it became common. The result was... unpleasant, to put it lightly.
 * The first four .hack games do this to nice effect by having a standard dungeon theme and a battle theme that more or less match up, and have one fade into the other when a battle begins or ends. It's especially pleasing in the ice-based dungeons
 * This is the point of the soundtrack for Fable 2: in fights, the music becomes more intense as you do cooler things; be a big enough Badass and you'll hear a symphony, more or less. Oh, and the cooler you fight, the more experience you get.
 * Played with in Tales of Phantasia. Morlia Gallery has about 10 levels...at first. If you choose to come back later for the Bonus Dungeon, however, the music gradually becomes lower and more distorted as you venture deeper. You can hardly make out the original theme by the end. Everything in that dungeon enforces a sense of paranoia and suffocation.
 * The cave/dungeon music in Dragon Quest also got lower in pitch as you went further down. It becomes Nightmare Fuel when you reach the lowest level of the Dragonlord's Castle.
 * Jeremy Soule is a pretty prolific video game music composer. He's done Total Annihilation, Neverwinter Nights, Elder Scrolls: Oblivion, etc. Usually his songs are composed in odd time signature (i.e. not 4/4 and not 3/4), presumably because it's more difficult to identify where the beat lies exactly, and thus when the game engine switches from one song to another, there's no jarring sense of a song being interrupted in mid-bar.
 * Resonance of Fate has two combat tracks for each battlefield type: a fairly laid-back version for normal combat, and a more upbeat one that kicked in when a character performed a hero run. The second version would remain in effect until a minute or so passed without consecutive hero actions.
 * Mario & Luigi: Bowser's Inside Story: There are two tracks for everywhere in the game, one for when you're walking around outside as Bowser and one for when you're inside Bowser.
 * Dragon Age II will play a sustained quavering note from violins as party members are killed and the controlled member loses health, while also muting music, fading between the two. By the time your chosen member is the only one standing and nearly down, the music will be completely mute and the violins will be all you hear. It is worth noting that the violins will still fade in at low health even with music turned off in the options.
 * Ultima Underworld had a soundtrack that followed the action during fight sequences. Ambient music would cycle along until the player took an aggressive action or something attacked, and then the combat theme would come in quite abruptly, both to cue the player that they were in a fight and to ramp up the adrenaline level. There was even a modification to the fight theme that changed the music when the player's health got dangerously low to tip the player off to heal, run or push for the victory before being killed.
 * Under normal circumstances, having an enemy appear in Final Fantasy XIII-2 will cause the music to switch to an 'aggressive' version. Other times there's just one track, period, and a few areas have completely different music for battles.

Shoot'Em Up

 * R-Type did this once: in Delta, when your ship goes underwater, the music changes to a more subdued, muffled version of the level's soundtrack, then goes right back to the original when you get back in open air.
 * Final does this in some levels, for example, techno-industrial percussion is added to the ambience when the mecha Mini Boss appears in the first level, and in the Giant Warship level, the music gets faster and more epic during the final approach to the boss.
 * Ray Crisis selects the musical score depending on which stage you start with. Each music track has a different variation for each stage and boss, looping if necessary. On the soundtrack CD, the variations are combined into 10-14 minute long suites. The Final Bosses each have their own music.
 * In Go Beryllium!, a Bullet Hell freeware game based around the subatomic world, the music adds unique beats: if you are firing, if you are firing in focused mode, if you have one of the special weapons active, if one of the bosses is onscreen, if you are hitting a boss, if an enemy is exploding, or if you die.
 * Inverted in Beat Hazard where the game changes depending on the music.
 * The thirteenth Touhou game, Ten Desires, has two different tracks for each boss and stage theme: one that plays normally, and the other when you enter a "spirit trance mode".
 * A rare NES example: In Battle Formula / Super Spy Hunter, each stage has two variations of its music, the transition occuring halfway through the stage.

Simulation Game

 * In the Harvest Moon games, the music track changes according to the seasons, with one track for spring, summer, autumn and winter. there are also howls at night, and crickets in summer in some places. In several games in the series (most notably the Mineral Town games), if you are low on stamina and turning blue, the theme becomes slow and creepy-sounding. Also in some games, the BGM will get deeper and slower the lower you get in the mines.
 * This occurs during Hank Freebird's operations in Trauma Team. The operations start with a base soundtrack that adds more instrumentation with each successful series of combos you perform. A single screw up causes the last set of added instruments to immediately drop out.
 * The Descent: Freespace series did this pretty well. Aside from transitioning to action music when enemies arrive, fanfare music is played when allies warp in or objectives are completed.
 * Done terribly in 1503 A.D., where every time the player shifted views to another inhabited island, the music would change.
 * In Animal Crossing: Wild World, each of the rooms in the museum slightly changes the tune.
 * Creatures 3 blew a fair portion of its budget on dynamic music. The music changes based on the land types, the creatures, the relative mood, and the amount of various objects (even food has unique altering).
 * Wing Commander had action-sensitive music which changed as the player fought against enemy ships.
 * Freelancer has this depending on whether there are enemies nearby.

Sports Game

 * SSX 3 did this pretty well: each event would have a single track play thought, with sections of the song playing and looping in certain segments, sound effects tied to the song that came on when the player successfully executed a trick and, the most apparent, the music almost going silent whenever the player hits big air, with only the sound of the wind and a VERY muted section of the song playing. When the player would wipe out, lyrics would stop until they got back up to speed or did a decent trick.
 * Pretty well my ass. They played the chorus lines for songs over and over again, and that was if you were doing well. These were not pleasant background videogame songs, these were actual licensed songs with lyrics. It was annoying if you hated the song and wanted it to end or liked the song and wanted to hear the entire thing.
 * Also notable in that the song would occasionally skip around, as the game would want you to cross a finish line around a specific point in the song, so at a certain distance from the end, you might notice an obvious skip. However, the player was allowed to pick which songs they wanted to play.
 * The original SSX did this as well, albeit more subtly (just varying the levels of each instrument). Added up to a surprisingly beautiful Crowning Moment of Awesome when you completed the main game and unlocked the 'Untracked' stage, a free ride down a pristine mountain peak with Finished Symphony by Hybrid playing in the background.
 * The BGM speeds up during the challenges in Backyard Skateboarding.

Stealth Based Game

 * Metal Gear Solid 1, 2, and 3 have two versions of the Alert theme, one for the "Alert" phase and one for the "Evasion" and "Clearing" phases. They are alternate versions of the same theme, and fade between each other seamlessly when the phase changes. An experimental "Making Of..." compendium called The Document of Metal Gear Solid 2 allowed users to (among other things) segue between the moods of each track at the push of a button. (For the curious, compare and contrast the Tanker Alert music with the Tanker Evasion music. As you can hear, the claustrophobic Evasion theme is a stripped-down version of the passionate, exciting Alert theme with the same chords - so if Snake gets seen again, the melody of the Alert theme can cut straight in.)
 * There's also the "suspicion" theme, a more tense version of the main stealth theme, played when a guard notices something, before going to full alert, ala Splinter Cell.
 * Also used a bit in some other situations. The music in the Shell 1 Conference Hall in Metal Gear Solid 2 gains a creepy string track if Raiden takes off his disguise, and an electric piano line is added to the music when Raiden is leading Emma by the hand, and a drum track when he's trying to snipe her pursuers. In Metal Gear Solid 3, several of the boss tracks change depending on what the boss is doing - instrumentation changes when The Fear is hungry, when Ocelot is breaking from the fight to reload his gun, when The Pain is shooting Bullet Bees, when The Fury's suit gets ripped, and so on. The vocal track only cuts in while fighting The Boss if you sustain nearly to the end of the ten minutes, and before that, you only hear the instrumental backing.
 * Because of the importance of listening to your surroundings, the Thief games tended to avoid background music of any sort... until they wanted to make you jump out of your skin. The very first level of the first game had one notable moment where you would be following a side hallway, and once you got to a more central passage, the game would suddenly play a single deep, loud note. DOOOOOOM.
 * Splinter Cell series: Arouse suspicion, a Scare Chord plays and the ambient music gets tense. Once on full alert, a techno track kicks in.
 * Even cooler, in the third entry Chaos Theory, there was a mission where you had to defuse time bombs. The beeping of the timers was integrated into an increasingly frantic techno track as they counted down; ie Mickey Mousing. Worth getting blown up just to hear the whole thing.

Survival Horror

 * Used a lot in the Silent Hill series, sometimes there's a variation of a music piece that only plays when entering or exiting a room(like the scary music when you return to the clock room for the last time in the second game, and the Scare Chord when you jump across the alleyway), or different variations are used for different types and numbers of enemies (e.g. the music quiets down when there's only one or two left), as well as different rooms(The alternate mall in the third game is a major example).
 * The music often gets scarier when you're about to enter the Dark World, such as after you unlock the school's clock tower or activate the hospital's generator.
 * Some ', for example the sewers in the third game, have up to 10-15 variations.
 * A particularly Nightmare Fuelish example occurs when you make the final Leap of Faith from the prison morgue to the Labyrinth in Silent Hill 2. First there's the regular droning ambience, then it adds a groaning noise similar to a steam valve, then a horrible high-pitched siren-like sound (so bad you have to cover your ears, worse than the "radio squeal" in the first game).
 * The series' best use of this trope has to be the alleyway sequence in the first game. First quiet ambience, then the air-raid sirens start up when it gets dark, then a loud scraping mechanical noise when you pass the gurney, then it adds an Ominous Pipe Organ when you see the blood and disemboweled corpses, and finally the percussion kicks in when you hit the end of the alley and get ambushed by the Gray Children.
 * Nemesis's Leitmotif in Resident Evil 3 had a suspense version when he's in the vicinity, and the battle version when he enters the room. There's a different battle theme variation for him after he turns One-Winged Angel, although it's still preceded by "Feel The Tense".
 * Resident Evil 2 was originally going to have more dynamic music, according to videos of RE 1.5, where the music seamlessly changed depending on the action and the player character's health.

Third-Person Shooter

 * Dead Space has a fairly revolutionary dynamic music system; the background music would rise as you walked through a room, and hold if you stopped.
 * By far the worst, though, is that the music sting that would play when enemies were approaching would not be played unless the player was looking at the creature. As a result, it's entirely possible to wander around a room, thinking there's nothing in there, and get stabbed in the back by something that's been stalking you.
 * Complete with a nice little music sting timed just right to go with the stabbing.
 * Syphon Filter series: Most levels have an "infiltration" theme and a "danger" or "battle" theme, which are dynamically segued between. Omega Strain also has "suspense" and "boss" variations for certain missions.
 * Naval Ops series of games featured variations of this. In Warship Gunner, proximity to enemy ships triggered the BGM to change from a naval theme to a low volume, tense repeating beat that would expand if the fighting went longer. Warship Gunner II featured varying BGM that changed from the main, rather unremarkable track to a more appropriate combat track.
 * In Resident Evil 4, creepy droning atonal music starts to build up when Ganados or other danger are approaching.

Turn Based Strategy

 * In the original GBA version of Yggdra Union, changing your in-battle tactics to Aggressive or Passive would change the tempo of the music and alter its mix slowly. The PSP rerelease has a similar mechanic, but restricts it to the player characters and only loops the first half of each Leitmotif. Too bad that there's no option to actually trigger the glitch that occasionally makes Yggdra's theme play sans bass in the GBA version.

Turn Based Tactics

 * X-COM: Interceptor's take on this was to use 5-second snippets of music played randomly and in succession during space battles. When something good or bad happened in the battle, like destroying an enemy or taking a friendly casualty, it would interject a "happy" or "sad" snippet of music into the stream. When the battle was over, the fast-paced battle music would change to something a little more spacey and ambient.

Visual Novel

 * The 2nd Ace Attorney game transitions from "Pursuit ~ Cornered 2002" (where the player character reveals what really happened) to "Hotline of Fate" (where they back it up) and back work like this.
 * As does Apollo Justices perceive system.

Wide Open Sandbox

 * Red Dead Redemption is, with the exception of a couple of songs, scored like this. The game is always in A minor at a fixed BPM, but drops backing instruments in and out depending on what is happening in the environment, which of the three areas the player is travelling in, and what the player is trying to do in those areas. Usually, bass riffs indicate combat, and whistling melodies means you are preparing to do something awesome (like raid a hideout).

Theme Parks

 * This is not strictly limited to video games. This technique is used in a lot of rides at Disney Theme Parks. For example, the "It's a Small World" ride has speakers throughout the course, with each set playing a different arrangement of the theme.
 * The Haunted Mansion and Ariel's Undersea Adventure use this too. Hidden speakers nearby various show elements (i.e., the singing busts, Sebastian, the Graveyard Band, or the saxophone-playing fish) will play a synchronized loop of just that element.
 * Some pinball machines also use this. (Specific examples, please.)