This should help
http://grok.lsu.edu/Article.aspx?articleId=12728
He is looking for a way to fade out the music at any time he want with a button.
This is just to fade it out on the end of the song and fade in on the begin of a song.
Goanna give it another try. I like coding

EDIT:
Code
#IfWinActive, Windows Media Player
::ControlSend,, {f9}, Windows Media Player
::ControlSend,, {f8}, Windows Media Player
I found this. Lowering the volume with F8, higher it with F9. Its not exacly what you want but its something. I'll keep searching.
EDIT:
After doing some more research.. Its not easy at all to make the music fade out. I found some AutoHoyKey programs for it but I don't understand them.. Guess I'll keep it to CoH2 codes
