A little more trouble shooting is required on this. Have you tried it with a Bluetooth headset to see if your voice transmits? What about on speaker phone? Lastly try a wired headset with a microphone. If your voice can be heard on all three of those methods, then most probably you have dirt or debris in your microphone hole. I'm assuming that the phone has never been worked on. Try using a dry tooth brush to clear the holes at the bottom of the screen of debris.

If the phone works fine when the screen is not installed in the frame but boot loops when installed in the frame, that is because you have some broken wires in the flex cable that when bent are causing a short circuit. The phone is boot looping because it is recognizing the faulty screen. Get a new screen and it should solve the problem. A good way to test if it is the screen is, if your old screen still works and just has broken glass, reconnect the old screen and install it and see if it works. If your phone boots up properly with the old screen, then you know you have a bad screen.

No that is not the digitizer. You can see later in the video he is still using the touch screen, therefore, the digitizer is still intact, otherwise the touch functionality wouldn't work anymore. It appears that is a layer of adhesive that holds the glass to the digitizer/LED.