Musical Gameplay

In a video game with Musical Gameplay, the sound effects are the music. Or at least contribute to the music.

In games with a Variable Mix, the background music subtly alters in response to certain events. This can perhaps be considered the ultimate conclusion of that philosophy - every event, from hitting an enemy to taking damage, produces not a sharp bleep or a hiss of static, but a clear musical note, chord or flourish. As the game is played, a rich tapestry of music is created, unique to this particular game session.

But, Musical Gameplay doesn't occur in what is an otherwise silent void. Usually, there is a simple backbeat playing as a conventional background track, to provide a rhythm and context to the musical sounds.

Compare Mickey Mousing, which is a similar idea applied to non-interactive media. See also Variable Mix, in which the sound effects are normal but the background music smoothly changes with the action.

Examples of Musical Gameplay include:

Action Adventure

The Legend of Zelda: The Wind Waker accents Link's sword-strikes with musical notes in time to the score, so that a fierce battle can actually generate its own melody.
- Same goes for The Legend of Zelda: Skyward Sword.
The Banjo-Kazooie series have this. A notable example is in Gruntilda's Lair in the first game, where the instruments used to play the Lair theme fade in and out when you approach the level entrances; Approach the desert world and the xylophone fades out for a snake-charmer's flute, approach the industrial world and you get horns and whistles.

Action Game

Taz-Mania for the Genesis varies the amount of instrumentation in the music depending on what is happening on-screen. If Taz is standing still, it will be sparse, but moving adds more percussion.

Adventure Game

Similarly done in Chibi-Robo!, as all of Chibi-Robo's motions are punctuated by different musical sounds, including walking, scrubbing floors, and shooting a blaster. An interesting note on the walking: the sounds change depending on what surface the little guy is walking on.
In King's Quest VI, when Alexander climbs a cliff, each step is punctuated by two notes with an overall rising pitch, and when he momentarily loses his balance, a teetering little melody plays.

Driving Game

Need for Speed Hot Pursuit had a subtle version of this: Whenever you wrecked particularly hard or hit an obstacle head-on, there would be a cymbal crash at the moment of impact, followed by a brief section where everything but the bass was cut out of the music as your car reset itself. The music also gets more upbeat when you are winning.

Fighting Game

Def Jam Icon uses the music of whatever artist you're controlling to distinguish who has the upper hand. People who have the music on their side are given a whole plethora of attacks from the environment, which, subsequently, reacts to the beat of the music. And boy, does it ever.

First-Person Shooter

In Left 4 Dead 2 the music changes when certain special infected appear, giving you a chance to adjust your tactics. Unfortunately between the gunfire, sounds of melee weapons and your three other party-members giving tactical updates, these musical changes are very easy to miss. And then there's a tank beating you to death.

Platform Game

In De Blob, when you paint buildings, the background music gets a brief solo, the instrument depending on what colour you are. For example, paint a building green, and you get a flourish on the piano, but Red might bring out the trumpet.
Super Mario Galaxy does things similar to the Wind Waker example above; the game has a normal soundtrack playing at all times (though several levels use Variable Mix), and every time a coin pops out of a block or a bush, its sound effect will match the backing music in pitch and tempo.
- There's also a galaxy in Galaxy 2 that has platforms that alternate existing and not existing to the beat of the music.
  - Levels like this are also in Super Mario 3D Land.
- There are platforms in both games that toggle between blue and yellow as Mario steps on them. The sound they make is set to match whatever chord the background music is playing at that moment.
In the first Sly Cooper game, the boss fight with Mz Ruby completely revolves around the music. You have to act with the beat of the music to stay alive.
- As well as this, whenever in stealth mode, violin strungs are plucked staccatto in time with every step Sly takes.
In Donkey Kong Country Returns, there is a level called "Music Madness," which is a factory-based level where the obstacles are in sync with the music.
Jak and Daxter and its sequels did this. While in Haven City, the music is very ambient, but pulling out a gun will add a percussion line, getting guard's attention will make the entire piece start moving. This is done so naturally that one usually doesn't notice it until they're pretty far in the game.

Puzzle Game

Puzzle game Lumines performed this beautifully. The sound-effects made as you dropped blocks or cleared areas of the screen tied into the trance-inducing music, which changed constantly as you progressed through the levels. The effect became increasingly hypnotic.
Similar to Lumines, Meteos has a basic background theme for each level. As you make matches, a different riff is inserted into the theme, which changes with every level of a combo. This is done smoothly enough that it's hard to tell the riffs aren't part of the basic music unless you check in the sound test.
Xbox Live Arcade game Chime is a similar deal to Lumines, with sounds tying in to the music as you place blocks and make quads.
Groovin' Blocks gives you bonus points for clearing blocks by hard-dropping your current block set to the beat of the song.
Music Minesweeper does this.

Rail Shooter

On-rails shooter Rez uses heavily-sequenced themes that are affected by, and themselves affect, the gameplay. Firing a shot produces a drum tap, lock-on combos create a phrase of notes. Defeating a large enemy will produce a crash or similar, or even abruptly start a new bar/measure in the music. Advancing to a new area in the level is delayed to coincide with the start of a new phrase of the theme. Ditto for Child of Eden, the Spiritual Sequel.

Rhythm Game

The Bit.Trip series for Wii Ware. Bit.Trip Beat and Bit.Trip Flux are "rhythm Pong" games, Bit.Trip Core is "rhythm Missile Command", Bit.Trip Void is a "rhythm Bullet Hell", Bit.Trip Runner is a "rhythm platformer" and Bit.Trip Fate is a "rhythm shmup". Oh, and each game is about 1000 times faster and harder than you're thinking. And it's Retraux.
In Patapon, you play a rhythm in order to control your very own walking eyeball army who start singing along to your beat. However, if you don't keep the time, the Patapons start getting annoyed with you.
Nightmare Before Christmas Oogies Revenge has remixes of the original's songs start playing for certain enemies, like "Kidnap The Sandy Claws" for Lock, Shock, and Barrel, "Oogie Boogie's Song" for Oogie, etc.
Otocky has the main character shooting out notes which make sound, depending on the direction they were fired at and the current instrument the player has.
Any game where you are "making" the music (Guitar Hero, Rock Band, etc) will mute out the music if you stop playing and\or make off-key sounds if you hit the wrong input. Beatmania goes the extra mile by actually playing the sounds exactly when the button is pressed, so each sound will actually be early or late to the song in line with the player's timing.

Role-Playing Game

Pretty much the entire point of Mother 3's "Sound Battle" system. Hitting an enemy (or an enemy hitting you) causes small melodies to be spliced into the backing music, and timing your hits right allows you to deal powerful combos.
Ar Tonelico 3 has the R.A.H. system, in which your Reyvateils' songs change during battle based on your actions. This has a variety of effects on combat.
Pokémon Black and White have several areas where certain actions such as walking add music or percussion effects to the BGM.

Shoot'Em Up

Space Invaders Extreme, notably in the DS version. Firing and destroying enemies triggers musical sound effects a la Rez and Lumines. Collecting powerups, going into a Round, activating Fever mode, or going into a boss battle will more dramatically alter the background music. In some stages, destroying the boss will bring the music from a frantic state to a more relaxed mood.
- This is done more effectively in the sequel since the main game is not interrupted by Rounds or Fever start.
The Indie game Everyday Shooter has an electric guitar track in the background, with various other musical elements coming in whenever you shoot an enemy.
- And that's just the first level. Each different level has its own musical theme.
Zillion Beatz, a Shmup, has this as the main gimmick: the bosses' attacks are perfectly in sync with the BGM.
Inverted in Beat Hazard, where the BGM you choose affects the difficulty.

Sports Game

SSX games take this trope to a new (at its time of release) level, in which the in-game music would go to a slight change when the player would hit big air heights. Also, at the conclusion of a race, the music would come to a down-toning end.
- On SSX 3, all the music happens to be on a "Radio BIG" throughout the game. This can lead to wondering of "how did they predict all this big air or dramatic cave fly-bys in their songs? And how did they time it so it'd end when I got to the end of the level?"