If you have a Fox & Hound signal tracer, audio oscillator with a pick up wand, they make great audio sniffers! I use the wand for all sorts of troubleshooting when it comes to tracing any audio issue at all.
"Old engineers don’t die, they dielectric."
From: Sherrod Munday [mailto:firstname.lastname@example.org]
Sent: Tuesday, January 2, 2018 12:39 PM
Subject: Re: [CRTech] Troubleshooting from a schematic
... now is a white noise generator and only the smallest amount of audio from any of the 3 inputs leaks thru.
I have schematics, but the PC board is not marked at all, and I’ve spent several hours trying to determine the cause. I have access to a fluke scope-meter,
Would anyone here mind a look at the drawings and venture a guess as to what component(s) might be guilty?
Have you tried poking around with an inductive audio amplifier?
I'd start at the output and work your way back through the devices/components on the PCB looking for where the white noise starts and the good audio stops.