I'm trying to record both what's happening on screen and the audio being played by the applications. To my surprise, QuickTime did not record the sound. I believe it worked before in previous OS X releases, but that is not something that I use often so I have not bothered to try earlier.
What is the current situation on El Capitan ? Have I messed with something that prevents QuickTime from capturing the sound, or has it never worked ?
Here is my view of Quick Time / New Screen Recording on a Mac Mini:
If your internal microphone is glitching out and you just want the system to recognise the hardwares presence again; try simply plugging some headphones or something into your 3.5mm TRRS audio jack.
I had the same problem the other day and thought the actual soundcard was fried but just making that quick contact brought it straight back online.