My microphone has been very "moody" lately, and it only decides to work when it wants to. Problems started to occur probably about 1-2 months ago, and I noticed them when phone calls were not going through. I'm unable to hear callers and the callers are unable to hear me. Also, calls and FaceTime calls just tend to drop and are unable to connect. More problems began to occur whenever trying to take videos with the camera app or with Snapchat. Videos are unable to begin recording, and when they do, there's no sound at all. I tried testing my microphone in voice memos, but voice recordings won't even begin recording. The record button senses my touch, but the recording itself stays at 00:00 and does not begin to record. I've tried factory resetting, hard resetting, etc. To try fixing this problem, but it only seems to be a temporary fix. Whenever I do these things, my microphone starts working again, but only for a few minutes before it starts to have these problems again. I hope this is only a bug and will be fixed in the next few updates and that it isn't a problem that will require me to replace my phone.
Read more: https://iphoneus.org/faq/iphone-7-microphone-not-working-on-ios-11-3.html