You need to download a .mp3 file, which is one of the music files FH reads, and copy and paste it into C:\FeralHeart\media\sounds, yes, sounds, not music, and then once you launch FeralHeart, go into Map Maker, and it will have a blank for music. That's where you put the name of the file, like, MyMusic.mp3, only you'd put the actual name.
If you don't get what I'm saying, try watching the video BouncyLion posted above.