Here is what I found testing on mine, and my sister's iphone (both 6)
There are 3 diferrent mics
-at the side of you back camera
-at the side of your front facing camera (at the same spot as the phone call speaker)
-another one at the bottom of the phone
When you are making a phone call:
-not at the speaker: the bottom one is used
-at speaker: all of them are used, sô if you cover one, the others two will be there working.
When you use the siri (i'm not 100% sure but i think that) the "front facing camera" mic is used
(For whatsapp users) when recording an audio, the bottom one will be used
When recording a video, the microphone used will be the one of each camera
My problematic phone:
-Front facing camera videos have no soubd but a *chhhhh
-siri doesn't listen tô me
-when in speaker mode (phone calls), if i cover both camera mics, the bottom one doesnt list to me too
-(taking those of, all the topics i mentioned before is working on my phone)
Here is what i concluded:
There are occasions that my front facing camera mic doesnt work (siri and video recording), and work in other ones (speaker on phone calls). And there are occasions in wich my bottom mic doesnt work (speaker on phone calls), and works at others (whatsapp audio, phone calls, voice memos)
All of this make me think, that this is not a problem with the mics. It could be a logic board problem, or a software problem.
I'll try to hard reset and reinstall ios using recovery mode on itunes. If nothing happens, its probably a HW problem