I had the same problem. Here is how I solved this:
If not already installed, install the PulseAudio volume control tool (sudo apt install pavucontrol)
Then, while Microsoft Team is running, open the “PulseAudio volume control” application.
Go in the Recording tab: you should see a mic volume slider for “Skype” (The Skype engine is probably used in Microsoft Team).
(If you don’t see “Skype”, be sure “Applications” is selected in the dropdown at the bottom of the Recording tag.)
Move the slider just a little to the right or to the left.
Then redo a Call Test in Teams. Now it should work.
After that, you can put back the slider exactly where it was, at 100% (0dB) and close the PulseAudio tool.

Source: In MS Teams for linux, microphone stopped working – Microsoft Q&A