I think Mike said he did break the input path while working in this and it was quiet.
So here's where I am at.
The audio path is from pin 20 of the CPU to the 10K to the left of it. That 10K goes to a cap in line with pin 4 on the amp IC. That cap connects to a 120k going to pin 4 on the amp IC, one of its 2 inputs.
So in series from pin 20 is a 10K, a cap (value?) and then a 120K to the amp IC pin 4.
The other input pin 3, has a 130K resister to series cap to ground.
I can measure the resistors but the caps don't have a value on them?
Using chipQuick, I removed the 10K SMD resistor from the CPU pin 20.
With my ear on the speaker, I could hear data buzz, and now some CFL hum buzz at my bench. Touching the empty lands really introduced buzz/hum as would be expected. This now is an open high impedance trace running around the board. No loading to ground except the audio IC input I suspect has high fixed gain.
Putting my finger across the vacant resistor pads, a played audio file would get through at a low level with Hum.
What I did do next was to tack a wire to the close by ground pad, from the left side of the now vacant resistor pad, closest to the left side of the board, that goes to the audio IC input circuit. To my ear, all but a very very low data noise went away on the speaker!
I have several audio files and haptic assigned to switches for testing.
I'm in a very low noise room where I hear a clock ticking at the other end! I'd call that quiet for most!
My thinking is that the path and the fixed gain is what's going on. To be honest, I fired up my Taranis in this room and it's pretty noisy. I'm again wondering how critical we're being?
I'll let this info be read before any fixes are tried. I'd like Mike to think on it and weather the input impedance needs to be lowered and if possible, more driver level how ever that's done? Not my area of knowledge FW wise.
Wow, those parts and pads are small! I'm glad I sold off a few planes and bought this awhile ago!
I think I remember Mike buying one that gave me the idea.