And success. It's back working.
I pulled the ribbon cable between the CPU and the Audio Board. When I was pulling it, I noticed that the cables have black "smudges" like someone took a magic marker to them on the CPU side. The audio board connector was reversed - the black was on the audio board side. Weird.
I took it to my bench, and using the leads left over from the R74 2.2K mod, I checked each trace from end to end on the connector.
Everything checked out less than 0.4 ohms except 1 trace. I tried cleaning it, re-clamping it - nope, it was running near an ohm.
In any case, I flipped the cable around so it was black side towards the CRT board, tucked the wires in, and viola! The audio is back working, and is MUCH quieter.
@toledoflyer for the win!
So the R74 2.2K resistor mod works. And now my audio is back online. I ran up a score of 3464 and called it a night.
I had an aircraft master solenoid I had to do a post-failure investigation next.