I haven't tried GM mode but I have placed several music volume triggers playing a few different music tracks fine in my editor and they work fine. Although not many tracks can be previewed by double clicking the resource in the resource manager. How are you trying to play the music?

For example I placed a MusicVolumeTrigger and set it to play "Explo_Theme_07_mystic". Make sure to set the bounds that the music plays in also.