Hi, Did it happen out of nowhere? Or did it maybe happen after your phone got wet or something? Have you already tried restarting the phone by holding the power button for about 20 seconds? If your phone didn't get wet and the restart doesn't help, maybe the LCD connector got loose. It once happened to me with my one plus 2. You can click the connector back into place by following this guide. Check if the connector on the right is properly clicked into place. Hope you found this helpful!
Hi David, No, the 6s and 6 have different home button flex cables. They look almost the same though. You can read about the difference in this older iPhone 6 vs 6S home button. Hope you found this helpful!
Have you tried recording a voice memo? If you record, a voice memo make sure the little speaker symbol in the top right is white (turned off) when testing the earpiece microphone. This means the Phone will only record using the earpiece microphone. If you can hear yourself clearly, you know it's a software issue. You can do the same with the speaker symbol turned on so that it records using both microphones. If the audio is bad, you know it's a hardware problem. Hope that you found this usefull, and let me know the results. I might be able to help out Justus Darley www.jfixit.nl