Your own music in the game

In the folder Sound of your Singles directory are the music files, which are played during the game.

backgroundSinglesMix.ogg
shopDrifters.ogg
singlesTitleSong.ogg
radioGitklirr.ogg

Music during game
Music in the buildmode
Musik in the main menu
Hifi

So if you want your own music in the game, you just have to convert your songs in the ogg-format, rename it and copy it in the folder 'Sound'.
As usual you should make a backup of the original music files.