Two things you can do..
1. Get Mame32FX which has volume normalization
2. Get my utility called "Command Line Volume" from
hereMy program is a little hard to understand, but what it does is you run it before a game using the usual launch before and after features that most FE's have. When in the game you adjust the volume to the way you want. When you exit the volume will be saved so next time you run it the volume will be the same. Unfortunately it doesn't work on a percentage of volume, so it will be the exact same volume not relative to the current volume. I've been meaning to change it to work relatively though.