Just posting for the sake of anyone in the future with the same problem.

I have a Williams World Cup. System 3 Type 1 Sound board.

It was working fine then no sound at all. Replaced all caps. Sound came back but then went out again and now a loud hum. Now no sound even when pressing test switch.
Replaced the Pia. No change. Replaced the Sound Prom with an Eprom and did the board mods so it could use a 2716 eprom (Jumpers, 10k resistors, etc). No change.

Replaced the Cpu. We have sound back but with a hum that get's louder over time. Eventually sound stops working again.

Put back in old Prom and put board back to the way it was. No change.

Replaced buffers (MOTOROLA MC14050BCP), sound is back and hum is totally gone.............................for now

Well hum came back. As it heated up the hum would get a little louder. Used cold spray and found the 7805 at fault.
Replaced it and no hum for over 3 hours.