If people in a meeting can't hear you, you might be muted or you might not have configured your mic correctly. Some devices have a physical mute option on the device itself, so check that your headphone boom is not in the mute position (usually upright or away from your mouth), or that your external microphone's physical mute button is not turned on.

To change your speaker, microphone, or camera settings when you’re on a call or in a meeting, select More options  > Device settings    in your meeting controls. Then, select the speaker, microphone, and camera options you want.

Troubleshoot microphone issues on a Mac

If you're using a Mac and you're having trouble getting your microphone to work, you might need to change one of your privacy settings. 

Here's how to check:

1. Open System Preferences

2. Select Security & Privacy

3. Under Privacy, choose Microphone on the left

4. Make sure Microsoft Teams is selected

Full article from Microsoft: Manage audio settings in a Teams meeting - Microsoft Support