Apparently there is a problem with the latest (1.3.3) version of Mumble. There is no input at all (the input bar is not blinking), I can’t speak! Problem only in Mumble, and many of our colleagues (from many locations, all with Ubuntu 20.04 or 18.04) experience this problem.
It seems like there is a problem between pulseaudio and snap, but not with other snaps. I test Jitsi meet in Chromium snap and I don’t have any problem. So probably due to the latest update 1.3.3.