Did you ever get this resolved? If so, what did you find?

I just repaired a batch of 8 of these style boards. On all of them the small blue axial electrolytic caps were bad in addition to the two large 4700uf caps. I think some of these were out of reimported games and most had at least some blown tantalum caps. I typically replace all of those too now.

Actually the WPC sound cards are supposed to boot when powered on the bench. The can also be reset by the main MPU of the game it is in. That may be the reason you didn’t get a bong noise upon power up. I ran into a board exhibiting a similar issue. Occasionally I would hear a bong on the bench but not often. If I didn’t hear it I could bring pin 33 on the ribbon cable high to 5V which causes a reset then the bong would sound. But that didn’t explain why it didn’t do it every time on its own. It was a reset issue. Digging deeper I noticed the sound card has a 4584 instead of a 74LS04 like the schematic shows but that doesn’t seem to matter. The on board reset is done by a simple Resistor R27 470k and capacitor C31 .001uf but the values they choose doesn’t seem to be inline with any of the other reference designs I’ve seen using the 6809EP processor. Swapped out C31 with a .1uf axial ceramic capacitor. Now it boots on the bench every time. Would be interesting to hear if you still have that old sound card if you could try swapping that capacitor and see if the bong sound comes back.