My computer audio developed the Dreaded Realtek "audio device not detected" bug, and the 'headphone was unplugged' big, simultaneously. I already know this motherboard is cursed, so time to go USB. I happened to have a USB headset from 2003 which still works. Yay!
How is this related to making anything? It wasn't a
stereo headset:
...but is, now!
I had to remove R39 to stop it mixing the left and right channels, and move the red wire so it carries left-channel audio instead of lighting a LED.
Then I soldered on a connector from my vast dead-headphone graveyard and huzzah, mono headset becomes stereo line-output. My wireless headphones are happy again.