There are 3 microphones in the iPhone.
The bottom microphone is for Voice Memo and phone calls. The other microphones serve as error cancellation
The (top) front microphone is used for FaceTime calls, Siri and selfie videos with the front camera (FCAM). The top back microphone serves as error cancellation
The (top) back microphone is used for videos with the rear camera (RCAM). The top front microphone serves as error cancellation.
You may have two separate problems here, related to the damaged housing.
[product|IF269-010|Lightning Port]
[product|IF269-008|Front Microphone]
Otherwise, there may be something wrong with the audio circuitry on the logic board.