This problem is cause by a misalignment of the frontal microphone. On the front earphone grill, inside of the screen, there should be a sticker where the microphone sticks to. There is a hole off to one side (left in this case when the screen is open and in front of you)
On the microphone, there are 2 holes on the ribbon cable. One hole is to align with a plastic pin, and the other hole is on the opposite side and center of the little metal box (microphone) That hole in the center needs to align with the hole in the sticker to let the sound through.
Let me know if that helps! You can notice on my screen that there is a clear plastic over the sticker for the microphone that needs to be removed first for the microphone to stick. Also, do check if your microphone hole is blocked on the ribbon cable form the old sticker or dirt!
Best of luck :) You got this!