Audio issues are most commonly due to a participant in the video session having internet connectivity issues.
Are you hearing your voice come back through the speakers when you speak?
This is due to poor internet connectivity and happens because the other person's mic is picking up your voice coming out of their speakers.
The other user should wear earbuds to prevent their microphone from picking up your voice. If you hear yourself twice, the issue is on their end. If they hear themselves twice, you should put in earbuds.
Another possible solution could be that the other user's speakers are turned up too loud. They may need to turn their volume down.
Are you hearing a loud high pitched sound?
This is called feedback and is often due to speakers being turned up too loud or the microphone and speakers being too close together. It will also happen if you are in the session on your phone and computer at the same time with sound turned on.
Turn down the volume or wear earbuds to prevent this.
Is the sound not matching up with the video?
This is due to poor internet connectivity. Participants can refresh the browser and start again. If you're using a data signal on a mobile device, try connecting to wi-fi instead.
You can also refer to this guide for more ways to improve internet connectivity and video session experience.