I have 3 admin buttons on each side. Coin/P1/exit on the left then Pause/P2/Coin on the right. I find this is just enough, but not too much. I did not want to use the shift function because I have small kids that play. No matter how well you hide the key press for the admin shift function, there is always a possibility that they will find a way to get in there. I tried to make my cab as "fool proof" as possible. Also by having separate coin, start, exit buttons it helps new players learn the system easier instead of having all these special combos to press.
As for sound volume, since I do not use the shift function for admin stuff, I have a dedicated keyboard tray which pulls out easily. I simply have my computer speaker controls in there with a piece of velcro attaching it to the keyboard tray. I find only certain games require I lower or raise the sound. If you use the shifted function, MAME has built in volume control that you can map to a shift function if you like.