Like @theimedic said, devices using iOS 10 are prone to a sound bug that was supposedly fixed in iOS 10.2.1.
So if you are not yet at 10.2.1, update or even better DFU restore.
If you still have an issue, it could be the two transceivers. It would be a board level repair. I suspect it would be the Qualcomm WTR1625L RF Transceiver also mentioned by iMedic.
However, a detailed history of the phone can help narrow down the issue.