Maybe I'm stating the obvious here, but why not just adjust the volume to a nice mid level (say 60-75%) and then leave it? Most emulators and apps read the wave volume label to set their volume anyway. I've found windows to be very good about uniform sound levels throughout apps.
I find that even within MAME, various games (/drivers) seem to have different levels to some others. Occasionally you a play a game which sounds really quiet, then the next game blows your head off!
I know MAME has the sound level on-screen setting (in my case they are all set to 0db). The problem is to use this to get uniform volume levels you would have to first find the quietest game (that would stay at 0db), and then reduce the volume of any louder ones. I certainly can't be bothered to do that!
Having said all that I don't (won't) have a volume control on my cab!
