An old (2014) link suggests that it is possible to do a full video recording (video+audio) on a mac with Quicktime, using only free software.
That seemed a little bit too good to be true, and when I tried it out on my recently bought MacBook Air with MacOS 10.13.6 and QuickTime 10.4 installed, I wasn't surprised to see it go wrong. The main problem is that as soon as audio is turned on in a QuickTime screen video, it starts repeating continuously an unbearable sequence of (mostly strident and high frequency) sounds in the background which all but hides any other sound. And yes, I double-checked the "Use ambient noise reduction" boxes to be checked and the volumes to be set reasonably in System Prefs > Sound.
This background theme sounds a lot like a proposital handicap, to be removed by payable additional sofware. I googled and browsed apple pages, but did not find any real explanation.