Two different sound paths show up in demos: the sound the computer or tab plays, and the sound from your microphone.
System / tab audio
Use it when the page itself talks — a video lesson, a product tour with built-in voice, or an error beep you need on the recording. Tab capture is the usual place this appears.
Microphone
Use it when you explain the clicks. See microphone narration. You can mute mid-take from the floating panel.
Using both
Narrate over a tab that also plays audio when the lesson needs both. Watch levels so the page does not drown your voice. If you only need silence plus speech, disable system audio and keep the mic.