

Audio devices can have multiple modes or “profiles” that determine what they do.
For my headset I have:
For my internal sound card I have:
If I set my headset to one of the options that doesn’t have “+ Mono Input” the mic stops working and doesn’t even show up in settings and apps anymore. Same if I use the “Stereo Output” mode on my internal sound card. They must be set to a mode with both output and input enabled to work.
I can see this from “Sound” in my KDE settings, but you can also configure this in the “Configuration” tab of pavucontrol.
Environmental DNA, apparently.