There's a trick you could use.
Song1 will be the song you want to keep playing at the same volume.
Song2 will be the song that you want to fade away.
Convert Song1 into a sound file.
Have them both play and type this in when you want to fade the music:
1 FadeMusic 0
Please follow these directions to the mark (they should work), you can change Song2 instead, but that risks the fading of all sounds.
Hope this helps.
Beware the GrimMonkey