Hey Rc Noobs,
I have not had this issue myself, but I do work at a phone repair shop and we had a customer with the same issue. We replaced the earpiece speaker, the camera/prox/speaker flex, and tried both at the same time. We also cleaned out the speaker and everything to no avail on our side. Seems to be rare, but seeing someone else with the issue personally leads me to believe this could be an issue with the phones logic board. Possibly common enough for posts like this, but not common enough for Apple to make known to the public.
Best of luck!
-Caleb