This is unbelievable. I too have the same issue where my microphone works for all applications, and even works in Teams 1V1 conversations. However...
in Teams calls with multiple people, NO ONE CAN HEAR ME (sounds like I am speaking from the bottom of a well). Based on this forum, the problem seems to be common. I am using a new Dell Laptop issued by Uncle Sam. I have run this by IT, we have checked every setting, and have found no solution. It is a shame, because otherwise, I really like and prefer MS Teams to other apps.
It seems to me this must be some glitch or problem with the software, perhaps with an automatic volume control setting specific to calls with more than 3 people.
MICROSOFT... HELP!