I'm definately not too familiar with the fadeMusic command, but from reading posts of people who have had questions about it I've gathered a few things... sorry if it turns out to not be %100 accurate information, I havn't been able to test it all myself.
_variable1 fadeMusic _variable2_variable1 = The time (in seconds) over which the command will take effect. (if you put in 20, over the next 20 seconds the volume will gradually change)
_variable2 = The change in volume. A number greater than one increases the volume, and a number less than one decreases it. From what I've read, it seems to go from between 0 (no music) to 5 (large volume increase). I'm not sure if the variable is relative to the current volume or not (meaning: if you set it to 1, I don't know if there will be no volume change, or if it will set the music to volume level 1).
So, based on all of this, if you use a script or the On Activation field of a trigger and put in:
10 fadeMusic 5 then it should gradually increase the music volume to max over 10 seconds.
Or, if you put in:
5 fadeMusic .1it should gradually decrease the volume to a quiet level over 5 seconds.
0 fadeMusic 0would be used to instantly turn the volume of the music off.
Hope that makes sense, helps you out, and isn't just repeating what you already know...