Why there is no "internal microphone" in my QuickTime player screen recoding audio source?
If the microphone is not listed as an Input device in System Prefs, then that's a larger problem than just QuickTime Player: the OS can't access the mic at all. 

Assuming you've turned the Mac off and on already, you could try resetting the NVRAM.

You might see whether updating to Mojave 10.14.6 improves matters.

Beyond that, it's likely to be a hardware issue. Is it possible that anyone's opened up the insides for a battery replacement, or other issue? Perhaps they didn't put the cable back properly.