Quoted from FlipperNTS:
That is a fair question
I did check the pot and also put a meter on it and verified it was working properly directly at the pot and at the connection to the sound board.
10KOhm between the black and red legs, and varied from 10K to 0 when measured from white wire to black and white wire to red.
I felt guilty even typing it... but I've had many friends new to pinball ask me how to adjust the volume and even found a machine I passed on where the speakers were stuffed with fiberglass cause they didn't know about the volume pot lol
I'd probably look at the sound board, remove (and reseat) the eeproms and inspect for any corrosion. Maybe examine the other connections for cold solder joints. Bust out the logic probe (after verifying it is not a voltage problem first) and inspect the PAL chips?
This might be another stupid point to check... but I think you can disable the sound entirely as an option in diagnostics. Maybe check for an option like that? I'm not home to look at mine.
check voltages +5 volts, -5 volts, +12 volts, and -12 volts. Bad voltage regulator? Bad connections?
Is a good resource.