You cant currently add music, you could replace the current ones. Also the music is only random on a few modes. Each chapter has its own song.
Are there any way I add more music to the Random "Playlist" in the game? (no.. I don't want to use Winamp :P)
Although each chapter has it's own song, level 6 in the first four chapter's & level 5 & 6 in chapter 5 has separate music
the reason why people can replace music is games are programmed to load the files that are listed (i.e, the game is trying to find a file that isn't there.) the point is, the game doesn't care if the music sounds different
Like, let's say you have some music you like better then what gish has. Go ahead and replace it!
gish is designed to load music from the music folder, so if you replace a soundtrack, make sure it has the same name as the one you are replacing, and you'll be fine