there's 2 volume pots on those. one's like the master, that's mounted on the game board itself, and the other is like a remote, accessible via the coin door test panel.
I would have to believe you need to shoot contact cleaner into the one on the game board and wipe it back and forth several times. deoxit you can spray while the game is live, others you might have a risk of nuking stuff, so I would encourage either way to do this while the game is off.
if you're able to clean it up and have consistent sound again, make sure you don't drive the one on the board too high or else it'll come out sounding distorted.