A wasted trip to the Apple Store followed by a successful iFixit repair!
My wife's iPhone was having microphone issues. No one could hear her on Facetime or speakerphone and there was no sound on her recorded videos, but it worked just fine for phone calls or voice memos. I took it to the Apple store but her warranty was long gone and they only offered to replace it for what would probably be the same price to upgrade to the new iPhone whenever it comes out. I told the genius bar guy I would try repairing it myself and he actually told me that was a good idea and it would be pretty easy as the microphone was right there as soon as you take the back off. I told him again how voice memos and regular phone calls were working just fine, so wouldn't it be the second microphone that I would need to replace? He insisted - same as all of Apple's spec pages and most of the information you find online - that the 2nd microphone is only for noise reduction and I needed to replace the 1st microphone on the bottom. He did admit that my wife's symptoms are reversed from the usual symptoms of microphone failure, but that logic would go no further because the 2nd microphone is only for noise reduction and therefore cannot be the problem.
On my way home, still being pretty skeptical that the same microphone can work just fine to record a voice memo, but not at all for a video, I got out my iPhone, started recording a video and blew on both ends of the phone. When I played it back, blowing against the bottom of the phone was somewhat audible, but blowing against the top of the iphone was quite loud. Trusting this test, I ordered the power and sensor cable. I figured it would be a bonus to replace the proximity sensor as well because hers had been problematic off and on but never enough for the Apple store to be able to reproduce it on the spot and fix it while it was under warranty.
I had replaced my own home button before, so I had enough confidence and I went ahead and threw in a home button for her since hers was starting to show the same symptoms.
Having done it before, I was able to get through the repair with no mistakes and only one dropped screw (hardest part of any repair). The trickiest part was trying to keep the 'sensor shroud' (little square piece on the back of the proximity sensor that had to be transferred to the new part) in place since there didn't seem to be much of any adhesive used to keep it in place. Between that and the earpiece speaker that kept falling out, getting the power button piece into place right required some serious tweezer dexterity and was the only step of the whole repair that took some time.
The best part, obviously, was powering on the phone after the repair was all done, testing the microphone, and just being right about the whole thing. No money wasted, lots of money saved.
Something I thought of *after* doing two of these repairs. For each step that involves removing a screw, take a piece of scotch tape and either pick the screw up with the tape or use tweezers to stick the screw to the tape by it's head so that the threads are sticking straight up from the tape. Fold one end of the tape over just enough to make a tap that you can label with the step # and the color code. Then when you come back to that step on the reassembly, carefully pick the tape up and use it to position the screw in place. Give the screw a turn or two through the tape until it bites and then peel the tape off. Not sure if that work so well for every screw (especially not the ones with washers) but it seems to me that would go a good ways towards preventing dropped screws.