Here is the scenario, you are using QEMU/KVM and are using something like the AC97 sound driver to pass the host audio to the guest via pulseaudio. This is useful because you can transparently pass your mic input from the host which means you can mute your microphone from the host, which prevents the guest from receiving any mic input even if unmuted.
Mute / Unmute Fix
This issue also seems to happen even if you press the mute button on the microphone and then unmute,........
In QEMU 4 or higher you can no longer use the normal "-soundhw ac97" flag and it is much more complicated but here is a simple copy and paste on Linux that will just work:
-audiodev you have to use -audiodev to specify the driver and id
driver=pa
id=someid
-device you have to specify the same audiodev id you used in -audiodev and driver
-audiodev driver=pa,id=pa1 -devic........