Music fade out problem

Yea there is!

So you wanna use the command:

volume sound X S

X = volume level (0 to 100)
S = length of time (in milliseconds)

So basically you need to add a sound (or music) with the volume at 0 (0%), then you want to boost up the volume gradually.
volume sound 100 4000

That will bring up the volume to 100% in 4 seconds