Hi Mark, to mute your mic audio for just you (so that the remote friend still hears you), drag your local monitor slider (under “You”) all the way to the left. You’ll see “Monitor:muted.”

If, instead, you press the Mute button (turning it blue), both you and your remote friend will be unable to hear your mic audio.

If dragging your local monitor slider affects how loudly your friend hears you, can you tell us what you have listed in the Devices section? I’d be looking for a routing issue.