Can't figure this one out. Not even a hum from the top speakers, just nothing. So far I've tried installing new speakers & new amplifiers but that has done nothing. All my voltages are good on my boards. Connections are all being made from speakers to aux sound board. At a loss as to what to check next.

I have a logic probe. & no I don't know how to troubleshoot them but I'd like to! The tda2040 are good on aux power board, I thought those were the only amp that would cause nothingness but now that you mention it I forgot about the 3403 ic. I already replaced those a while back on the main board, but I do have new aux sound & aux power boards that I could install new ones on.

My old aux power board had 12v dc leakage to the speakers. I never could track down the cause. New (old) board from PBR is in there now. I actually do have an oscilloscope ordered since I found one cheap online a couple weeks ago so maybe it will come in handy here. In the meantime I may try new 3403 on the aux power board & see what happens.

I did that, & still no sound from the top speakers & an awful screeching type noise on the bottom.
Speakers are confirmed good via ohm test (also they are new, from a new stern game I removed them from), all wiring has continuity to the back of the aux power supply board pins, & I've replaced every 3403 ic on all 3 of the boards. Still having same issue.
Any chance a bad rom could cause this?
I burned some new roms for the aux sound board despite multiple attempts & using different chips I couldn't get AROM1 to verify. Wrote ok but fails verification. AROM2 didn't have that problem.