The bottom microphones is for Voice Memo and phone calls. (so is possible to be the lightning connector assembly - iPhone 8 Lightning Connector Assembly)
The (top) front microphone is used for FaceTime calls, Siri and selfie videos with the front camera.
The (top) back microphone is used for videos with the rear camera .
If the issue affects more than one microphone, then it could be the Audio Codec IC on the logic board. Anything at the logic board level will require micro-soldering so you would need to find a repair shop that offers those services.