Check your electrolytic caps on the sound board.
I know the caps in question on the two board set are C23 and C24 (both 4.7uF)
but the reference designator (cap number) is different on the four board (IIRC)
it may be C30 and C33 but I'm not 100% on that. I think there are only two caps that are
4.7 uF electrolytic on the sound board. Check those, those caps can break pretty easily as they are on the outside of the boardset.