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.
Related (but older, and perhaps a little outdated) questions : here, there
unbearable sequence of (mostly strident and high frequency) sounds
does not appear to be a description of anything musical. So far as I know, QuickTime does not add any propositional handicaps requiring the purchase of additional software, licenses, etc, to remove the handicaps. Something else is going on. I've used QuickTime to do many screen recordings and have never heard anything like you're describing. – IconDaemon Sep 25 '18 at 13:21