Check your resistor values on R22 through R25. R22 and R25 are supposed to be 120k, not 150k as listed in WPC schematics. R23 and R24 varied PER GAME, and TZ's values should both be 56k.

My guess is that yeah - since it seems to be per-channel, it's not in the end-amps or any common circuitry. And from past experience (where I had a sound board that sounded horribly 'tinny' with sound effects) I did some research and found out that resistor values were changed like that. Past that, I could check caps, yup. These boards are over 20 years old now.

Not a problem!
It would have worked in other WPC games, but some games like Terminator 2: Judgment Day had those value resistors as well.

According to my research, games with 56k in R23-R24 were:
Twilight Zone, Creature from the Black Lagoon, The Addams Family, Black Rose, Doctor Who, Bram Stoker's Dracula, Fish Tales, The Getaway: High Speed II, and White Water

WPC games that had 150k in R23-R24 were:
Terminator 2: Judgment Day, Gilligan's Island, Harley Davidson, Party Zone, The Machine: Bride of Pinbot, and Hurricane

It's also likely that resistor values were changed based on how loud the voices were originally recorded and digitized into the DAC sound format. But yeah, I'm guessing the folks that originally bought it didn't even know that wasn't how it was supposed to sound.