It's gonna be one of two things (probably).
The small amp board has three channels....thus three volume pots, three speakers. If one of the channels goes out, one volume pot stops working, one speaker is dead.
The other thing it could be is an issue on the "game board" pcb. This is the pcb in the card cage with the two connectors plugged into it. I would swap the custom chips at locations X2 and X3. These two chips are identical....and handle input, output, and audio. Swap these with each other and see if the sound changes.........if it does, you know one has issues.
Edward