Volume Control for Different Snap Videos?

[Larsa]

I have video snaps for MAME from EMUMOVIES, but some have soft volume, some have loud volume, and some have very loud volume.

Is there a way to control the volume for each snap movie (displayed during the MAME Wheel)?  For example, could you designate a volume level in the Theme for each game?

Speaking of which, is there a webpage/wiki/document somewhere that explains the controls available in the Theme.xml file? 

Thanks!