Confirmed the bad connection on pin #13 of the opamp IC. Didn’t have my soldering station handy, but I VERRRRY carefully bridged the pin and pad with a metal twist-tie I had lying around. Instant audio output. So that’s an easy fix 🙂
Also confirmed pin #13 “RLY 2” on the Atmega. The moment I connected pin #13 to R32, I heard the tell-tale “click” described in the troubleshooting guide that I hadn’t been hearing. Hopefully it’s the solder joint at R32 which should be easy to fix from the top of the board; if it’s the pin on the socket, I’ll likely have to go pulling it apart (unless I want to throw caution to the wind and solder a jumper wire directly from the pin of the chip to R32… which I don’t).