The music is playing out of the ambiance channel because the map maker did not specify that sound file (the music file) as being a Music group, so the default setting is to make it Ambiance.

Basically all they have to do is right click the music file in the editor, go to Group (or something similar, I'm not in front of the editor) and choose Music.

I worked with a BUNCH of map makers to get this fixed (see FeelGoodInc), because it drove me crazy as well. :)