Note: If Front Microphone works and Rear Microphone fails in PhoneCheck the device has a bad Rear Microphone
It will seem like the Microphone is working because the voice memo app will pick up on voice. Apple devices automatically switch to a working mic on the device to compensate for this failure.
PhoneCheck picks up on this switch and fails the Rear Microphone accordingly.
How to test and verify Rear microphone is bad
Take two devices of the same model. One with bad rear microphone and one that is working.
Record someone through the rear camera on both devices.
When comparing 2 of the same devices we found the bad one to be slightly lower then the good one.