Calm4.ogg

Calm4.ogg was a music file (alongside the other tracks) that was beta-tested and created by Notch himself.

It consists of a boss battle like tune, with a quieter part featuring what is now known to be Notch saying "Mojang Specifications" in a slowed-down voice.

The track was released around, but it is unknown when and why it omitted from the downloaded game files. As such, players who had the game while the song was still in it will continue to hear it being played, as the game will play any song in the .minecraft/resources/music folder.

See this video for more information.