If you cannot hear anyone
Check your computer's volume, output device, and whether the browser tab is muted. Reconnect your headphones if necessary, then leave and re-enter the room.
If you cannot hear a particular person
They may have their mic off, or they may be in a private talk you are outside of. Touch the group's camera tiles to join, or ask them to step out.
If a microphone object is in use
Only speakers on microphones are heard across the room. Private groups still hear their own conversation. Move onto an available microphone if you need to address the whole room, or wait for the speaker to finish.
If others cannot hear you
Check your mic button and device selection, headset mute switch, and browser permissions. Use the microphone troubleshooting steps if it is still silent.