I am pretty sure that classic Bally sound volume is not controlled by MPU.

It looks like there were eight different sound boards for bally pinballs, including the squawk and talk, vocalizer, say it again, cheap squeak, etc.

What game is he working on? More importantly, which sound board is he using?

I recently was working on the AS-2518-50 sound board for a Harlem Globetrotters, and was surprised to find that in addition to the 'play sound #1', 'play sound #7' lines, the board depended upon a basic tone pulse from the CPU. It made me wonder if the basic clock speed of the MPU were different, if it wouldn't affect the tones.

That having been said, the AS-2518-50 has two potentiometers, one for volume, one for sustain. These pots are notorious for having dead spots and they need to be replaced whenever you find the originals still on the board. Otherwise even microscopic vibrations can cause the volume and sustain to vary, seemingly by itself.

Anyway, we would have to know what sound board is involved before we could comment on what might be causing a difference in volume.

Okay, I have to ask, did he try turning to volume down?

Sound too loud, now that's a new one. Definitely would help to know the game or if the buyer knew how to adjust it.

The squawk and talk volume can be controlled by the MPU. It is almost never jumped to do so, but I've seen this in maybe one of the very late Bally -35 games.

Have them step through audits and change all audits past 15 to "03". You can look in the manual for what exact audits do the vol control, I think it is the last two.