I just picked up one of these with a similar issue. I have barely spent any time on it, but I was able to get the volume to pop back up by adjusting the pot on the sound board. The pot on my mine is clearly very flaky and needed to be turned slowly until I found a "sweet spot" that brought the sound back. I imagine I will need to replace this pot at some point, but for now it works.

Also, Bally put out a service bulletin urging operators to remove capacitor C12 as there is a power surge when the game is turned off that can damage a 7533 digital to audio converter chip (DAC). You may want to see if this modification was ever done.

http://www.pinrepair.com/6803/6803removecap.pdf