If you have a mini jack to mini jack cable, I believe you can plug one end into the line out port of the mac, one to line in and I believe you can record the audio that way if you select 'Line In' on QT's screen recording menu.
Of course, this means you won't be able to hear the audio unless you buy a splitter and wear headphones/attach speakers.
It also requires a Mac with both line in/out, which I'm not sure how many still have. I know 15" cMBPs have them.
[doublepost=1502487142][/doublepost]Selecting QuickTime's New Screen Recording opens a window with a Red button that starts the recording. Clicking the down-arrow just to the right of the Red button allows you to choose which microphone (if any) you wish to use. In FaceTime, select which camera and microphone you wish to use from the Video menu.