The first thing to check is the users section and emoji. for example..
Solution: turn on audio from bottom of screen by clicking the telephone and completing successfully the connection process. if problem persists check your web browser permission, FAQ
the second thing to check is if Mic is muted. Picture follows..
The third thing to check is if your in listen only mode, picture follows..
Solution: click on telephone on the bottom of the screen and successfully complete the connection procedure.
If you see a Green microphone, Picture follows
and you Do not have Sound or Microphone or both, check the physical devices, if they are turned on or connected correctly.