Sorry to dig up an old thread, but I finally found a spare PP pcb I had. I swapped 52's and 9c and no change. Garbled/muffled sound.
one thing I did notice is that the mostly working pcb is missing a socketed chip at 10a on the cpu board. On another PP pcb I have(converted to PPII?), the 10a chip is soldered into the board. It has a sn74ls128n in there. I assume that could be the source of my sound problem. Anyone know what it does? Need to find one and see what happens.