As in cases with music, often have to make copies and rename mp3 files and place them in music folder.
For instance, if was adding "The Last Stop" for Firefly Island, I would save the mp3 as firefly1 into the music folder.
Afterwards, would go back to main Feral Heart folder and open the music.cfg file.
In this, would put the following; (The keyword for my line would be Firefly, yours would be your own).
[Firefly]
media/music/firefly1.mp3
Save file.
Then would open up map maker, go to the tab that allows the music to be added and type in the keyword, (Firefly for me).
Would then click test. (Make sure sounds is actually turned up, have had someone explained this to not even have the music ON).
It should work, if done correctly. I've no idea if this is different from Rimfrost's method, has I haven't ever looked at it myself.