To get your microphone up and running, please close any other programs that may affect your webcam (do not just minimize). The fewer programs are open at the same time, the better.
You can also adjust the microphone sensitivity in the system settings:
Click Speaker > Recorders > Microphone > Settings > Level. You can now increase the sensitivity of the microphone.
Click the Apple icon > System Preferences > Sound. You can now configure the sensitivity of the microphone input.
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