I was trying to capture audio of a webinar and Quicktime was capturing the OS audio as expected, but for whatever reason it was not picking up the audio from Firefox. I decided that maybe it was a browser thing and I was right. Logged in to the webinar with Safari and it worked perfectly.
Download Sound Flower before trying to record.
Download, install, and then open. Once open, you will have a little icon at the top of the screen. Select Sound Flower 2 Channel, Built In Output.
File -> New Screen Recording
On the upside down triangle, select Sound Flower 2 Channel