Below SOURCES, select the icon, to the right of your Phone and/or Webcam/Mic source.
By selecting the icon, you're able to mute the audio for that source.
Once muted, the icon will display as .
As a reminder, this will ONLY mute the audio of the source. Therefore, if the source is in the scene and displays in the media panel, the audio will not be heard but the source will still be seen.