how to record screen video with sound but not playing while its recording

Hi, I'm trying to figure out a smart way to record some video lectures using my computer. I note that the Quicktime player is able to do that, but it confines your screen to a certain size so anything that falls into that screen capture zone will be recorded which is fine as i get a separate monitor soon so i can just chuck it one side to record while I'm doing other work.

But what im still trying to figure out is how the sound can also be simultaneously recorded by the software (be it Quicktime or not) and yet not be playing out loud on the speakers so i can do my work and not get distracted by that.

Any line-in or some clever way to do that?

Windows options also possible using parallels if anyone has a solution