1) Make the state of the “Send” button clearer, e.g., when the button is disabled the button could say, “Not Sending Audio”, and when enabled it could say “Sending Audio”. Maybe even change the color also?
2) For new users, have a “test” feature. User starts the test session. Site alternately records an Interval and then plays back what it recorded, showing which cycle (record/playback) is happening, so the user understands. This way, users can test their setup, and hear what they sound like before they involve themselves in others' sessions. Win/Win for all.
3) Since Settings are not editable when the user is in a session, consider making those windows modal and “in front”. As is, it's very easy to lose the settings behind the session window.
4) Or consider bringing Settings windows to the front whenever the user selects Preferences… from the menu bar.