Check that there is a microphone icon besides all the names in the Participants Panel.
Absence of a mic indicates that the participant has not Joined with Computer Audio.
Check that it does not have a red line on the mic.
On individual screen, check that your own mic Is not muted.
Click “Unmute on bottom left. Mic indicator should turn green as you speak.
Click the “^” button to re-select the Mic or do a “Test Speaker and Microphone”.