Oof. I use my single channel one a LOT. All those solder points look okay from here though I’m sure you can see better irl. I might go so far as to try and trace the circuits with a tester but it’s honestly crazy that both channels would quit given their independence.
Looking in mine, if there's any obvious weak link it's the solder connection to the ground lug on the microphone-side connector. this and the fact that the ground is the only thing both sides share is all I can really think of.
It is definitely beyond my abilities to diagnose (no obvious breaks in solder or loose chips etc) and it's very weird that both failed at the same time (though who knows I haven't used it in like a year so maybe the failed independently and it just took me forever to notice).
The ground would make sense! But it seems fine so idk.
Well that’s all I’ve got.