I have been looking for a fault on a CPS2 A board today. There was no volume coming out to the JAMMA connector but there was from the line out connectors (Stereo Q sound).
I couldn’t really find anything wrong as I had proved the on board amp was working.

I entered the test menu and it reported the volume was up full so I started to turn it down, when I got half way down the volume bar disappeared and the volume reported being set at 0. When I started to turn the volume up again the sound was back and it works perfectly.

Not sure what this was all about but its certainly something to keep in mind before I start poking around with a similar fault again.