TECH: Help! Pinbot sound woes: More like a blare than a hum...

I have been working on a Pinbot and have all of the backbox electronic
happy except the sound. When I connect the backbox speakers and turn
the game on, I get a horribly loud blare (much louder than what I
would consider a hum) out of it. I don't ever hear anything from the
cabinet speaker, although I am not sure that I have done the SW1 test
mentioned below (which sort of works) with only the cabinet speaker
connected.

I don't hear any power-up sounds or any attract sounds over the
deafening roar, but I am not sure if there are any, as I have not
heard a working Pinbot to compare. If I can withstand the barrage and
make it to SW1, when I press it I hear a loud PSSSHT sound like
letting air out of a tire and after a few seconds, I hear Pinbot say
"I See You," but not very loudly (the PSSSHT is pretty loud). Not
sure what I am supposed to hear during this test, but I am taking the
speech call as a good sign.

The blare seems constant regardless of volume setting or the state of
the displays. If it matters, I don't have the coin door/playfield
connected.

All screws are present and tight for the sound/CPU boards and ground
continuity checks out with a meter. -12V reads -13.5 and +12 is at
+11.5. +5V is about +4.85. I already replaced all of the
electrolytic caps on the sound board and no change. I have not
replaced the -12V cap on the CPU board nor C8 on the power supply as
noted in some of the 'hum' fixes I have seen, as the ripple on their
respective power connections looked OK.

This seems much worse than a hum, but maybe one man's hum is another
man's deafening roar.

Any advice out there?

Thanks,
--Mike B.
.

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