Well, first of all you need to find the music you're wanting. After that go into Local Disk(C:)\FeralHeart\Media\Music and you should see; caves1, caves2, plains1, plains2, plains3 and title. Finally, get the song you want and re-name it to one of the default songs already in the FeralHeart and replace if asked to.
Remember: Caves1 & 2 are played in cave-like places. Plains1, 2 & 3 are played in Out-side places and the title mp3 is played when you open up the game and you're on the login screen.