How to identify the cause of the microphone mute on Windows 10?

Question

Skype

In order
A laptop with Windows 10, working, it has Skype for Business. Don't ask me why, I don't decide.
Everything worked great until I had to make a presentation through SfB, with a tricky audio setting - speaker output, nikrofon via USB
After that, the regular microphone through the headset completely refused to work. The headset itself is naturally working, it was tested on another laptop. "Refused to work" - the device is fully detected and working, but the indicators of the incoming sound are at zero.
It was fixed by connecting a USB headset, and not the first time.
A couple of months later, after the update and when starting SfB, the microphone went out again. Reverting the update didn't help.
Tell me where to dig.