I don't know why it doesn't work.
Here is an example. Go to the Creation Code of the first map to see my code:
http://www.mediafire.com/?4faxxlrp5gkpb70
I tested out the sound volume, it turns out it doesn't work for mp3s, only midi and such. The music you had on the gmk. I might have something you can replace it with, but I need to find it on my pc first.
I suggest you only use mp3 music only for rooms that you need one song playing.
Plus here is a new set up for the music to play, if you have any midis
if !sound_isplaying(Hyrule_town){
sound_stop_all()//if you want to add another sound, add another loop sound right below the stop sound code.
sound_loop(Hyrule_town)
sound_volume(Hyrule_town,0.7);//possibly the lowest for a song/sound to play
}
remember, try to find a midi version of this song you need.
To get you started off, here is one that might suit you well. If I find any of those mp3s in midi, ill repost/edit.