I'm having the same issue running Teams Preview Linux client on Manjaro 5.10 (and 5.12). The microphone had been working up until a couple of weeks ago, but now I get the message "Your microphone isn't working". When I run Teams through my web browser (Chrome) the mic works fine. The mic also works fine in other apps such as Zoom.
I tried the "solution" above, adjusting the mic level for the app in the Pulse Audio app, but that didn't make any change in how it operated in Teams. Any other solutions?