One of the microphones on my wife's iPhone 5 quit working. No Siri, no Facetime, hit and miss speakerphone. I took it to the apple store to get their evaluation so I could make an extended warranty claim. They said they could fix it for about $60. The Genius bar guy knew it was probably the microphone, but the guy in the back that did the repair was "NOT A GENIUS". He said he replaced the receiver and that didn't fix it, so he would need to replace the screen. When I asked him what any of that had to do with the microphone, he didn't know, but just said that sometimes it fixes it. He wanted to charge me $125 for replacing the screen, so I decided to buy a replacement phone and leave. When I started looking on iFixit, I realized that the microphone was not related to any of those parts that the NOT A GENIUS mentioned, I decided to undertake the repair myself. After success, I returned the replacement phone for a full refund.
I had already taken the phone apart to investigate, so I knew what I was getting into, but the feedback from the forum to verify what I was suspecting was priceless. I was able to swap the part out easily and got the phone back together and all was successful!
There's a little black plastic spacer next to the front camera. As soon as you remove the shield it could come out, so just be aware of it. It's the size of a crumb, so you will probably want tweezers to get it back in place when reassembling. Good luck!