You can check your settings by clicking the three buttons in the top left hand corner of the screen.
If the green dots are animating, your microphone is working
The three dots will bring up the settings menu where you can change your microphone and video settings
You can also access the setting screen from the three dots in the right hand corner at the base of the screen once you are in the video conference
If all else fails, try logging out, then logging in again!