Slightly off topic but I have two button configured on my control panel to adjust the volume while in the Front-End, MameWAH, by using an AHK script that I wrote. I would guess you could edit the script and make it any two keys you like. That way you could use too "odd" keys (not used by the game) to adjust the volume while the game is playing. It may work. The script looks like so:
; This script assigns hotkeys of your choice to raise and lower the
; master volume.
;_________________________________________________
;________________Global Settings__________________
#NoTrayIcon
#SingleInstance force
;_________________________________________________
;________________User Settings____________________
; The percentage by which to raise or lower the volume each time:
vol_Step = 2
HotKey, m, vol_MasterUp
HotKey, n, vol_MasterDown
vol_MasterUp:
SoundSet, +%vol_Step%
return
vol_MasterDown:
SoundSet, -%vol_Step%
return
Good Luck,
John