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To work with the Audio Mixer:
What it is
What it does
Line-in audio channel
Video source channel
Program (live) channel
Other controls work as above.
Caution: This can cause severe feedback if the Mac, or any speaker(s) it is playing through, is near the video source's microphone.
Note that toggling the headphone icon to monitor audio doesn't change the muting/unmuting of source audio going into the program. To control that, tap the speaker icon.
By default, the audio mixer setting for any of these is AFV mode:
Meanwhile, by default, the audio mixer setting for any Line-in connection is Always On mode. Therefore, if there is a Line-in connection, by default it is the program audio unless you mute it by tapping the speaker icon.