First use of jitsi - Test sound worked, but no user audio on either side

I could see that there was a signal that was coming from my webcam microphone; the video of the two participating parties was perfect; but neither of us could hear anything.

While this was happening, I went and found the Menu > Settings > “Play a test sound”. I clicked that, and heard the test sound, but I couldn’t find any way to be able to hear the other person.