1. Click "Capture" in the right sidebar, click "Monitor", click "Refresh" icon to list  all the monitor of the system, select a monitor, and drag and drop it to the waiting window.

    The results are as follows:

  1. You can set the audio for the  source as needed.

In the attribute window of  source, click "Capture" and check  "Audio". You can capture  the audio from the specified audio device.