Music Disc

Music Discs (previously named Records) are items added in Alpha 1.0.14 that can be played in jukeboxes. They play music made by C418. It has been stated that in the future, there will be 14 music discs to find in Minecraft.

Despite the music files themselves being saved in a playable .ogg format, custom music cannot be used unless a modification is installed.

There are only two discs currently implemented into the game.

Acquiring Discs
Music Discs are found in 8% of dungeon chests.

Music Discs are also dropped when a creeper has been killed by a skeleton's arrow.

This can be achieved by positioning a creeper between a skeleton and yourself and having the skeleton fire at you. A creeper will target and attack a skeleton once the skeleton's arrow damages the creeper. Therefore it is important to ensure the creeper is only one arrow hit from death. Otherwise the creeper will simply explode and kill the skeleton and itself before the skeleton is able to fire another arrow. Shooting the creeper with arrows four times or hitting it with an iron sword twice will weaken it enough that a single arrow will kill it. Due to Beta 1.5, all swords do one half heart more of damage, so it is no longer possible to effectively weaken the creeper with stone or wooden swords. A way to arrange the final killing shot involves trapping the weakened creeper and a skeleton in a 2x2x3 hole, then walking around the edge until the skeleton fires and hits the creeper. Be sure to back up quickly if the creeper begins to hiss.

The Changelog for 1.6 confirmed that the record player will support 'more than 15 songs'; however, as no Music Discs have been added this feature is not yet used.

Gold Disc
The gold disc plays the track "13", an ambient track consisting mostly of ambient cave noises, metallic clings, splashes, wind blowing, and skeleton and creeper sounds.

Green Disc
The green disc plays the track "Cat", an upbeat synthetic melody.

Unused Music Files
Interestingly, there are 10 unused music files stored along with 13.mus and cat.mus (which have been decoded as 13.ogg and cat.ogg respectively). These might be the songs that Notch said will be added.
 * "11" - The sound of someone panicking in a cave. These were decoded by a mod.Citation Needed
 * "blocks" - Upbeat music.
 * "chirp" - Another upbeat, catchy tune, heavily inspired by the "LittleBigPlanet" theme song.
 * "far" - Calm, relaxing music.
 * "mall" - Another calm piece of music.
 * "mellohi" - A slow, creepy guitar piece.
 * "stal" - A moderate jazz-like piece.
 * "strad" - A tropical-sounding synth piece.
 * "ward" - inspired by "Heavy Rain".
 * "where are we now" - A synth-pad piece.

Trivia

 * Gold music discs were noticeably more common than green ones until Beta 1.2_02, but after the Beta 1.3 update green records are more often dropped.


 * The 12 music tracks created for records by C418 can be found in:
 * Windows: %appdata%\.minecraft\resources\streaming\ (AppData is hidden by default)
 * Linux: ~/.minecraft/resources/streaming/ (.minecraft folder is hidden)
 * OSX: ~/Library/Application Support/minecraft/resources/streaming/ (~/Library is hidden in 10.7)
 * Of the 12, only Cat and 13 are in a readable Ogg Vorbis format. The remaining .mus files are locked with a simple encryption algorithm. However, they can be decoded by Minecraft on the fly.
 * Before Beta 1.6, an SMP bug existed when a music disc was ejected from a jukebox. The disc would pop out along with a "ghost" client side disc that only you could see. This ghost disc was not able to be picked up.
 * You can make the discs play your own music by replacing the .ogg files for "cat" and "13" with your own .ogg files.
 * There is currently a bug where if you put a music disc in a jukebox and then turn the sound off (in minecraft options tab,) the music will continue playing, even when you eject the music disc. Regardless of how far you travel, you will hear the music until the song ends. This can be fixed by turning the sound back on, reinserting the music disc, and then ejecting the music disc.
 * C418 mentioned a new music disc for 1.8, but there's not much info about. And, as a bonus it wasn't included in 1.8. Or 1.8.1