A client cannot hear you, or you cannot hear them

Work out the direction first. It halves the problem immediately.

They cannot hear you

1. Check Mic is not crossed out.
2. Check the right microphone is selected, with the arrow beside it. A laptop often defaults to a microphone you are not speaking into.
3. Ask them to turn their volume up, and to check their own output device.

You cannot hear them

1. Open Participants and check whether they are muted. If they are, select Unmute and ask them to turn their microphone on: releasing them does not switch it on for them.
2. Open Status on their row and look at the microphone line. A blocked microphone shows there.
3. Send them Allow your camera and microphone.

Echo

Whoever is not wearing headphones is the cause. Their speakers are feeding their own microphone. Headphones at either end fix it; headphones at both ends fix it properly.

If neither of you can hear anything

That is a connection problem, not an audio one. See Calls keep dropping.