Check that you have joined audio.

Make sure you have the volume turned up

Try playing other audio on your computer to make sure your speakers are functioning normally.

Check that the participants’s microphone is turned on. In the User list and make sure they have a microphone icon on the right of their name which would indicate their microphone is on. If not, then send them a message in the chat box or type it on the presentation, asking them to turn on their microphone.