The microphone on my iPhone 6 won’t work
My Microphone works with telephone calls and Siri and voice memos, but it doesn’t work with FaceTime or when I make videos. Why won’t it work?
Take a look at the link.
Get help with the microphones on your iPhone, iPad, and iPod touch - Apple Support
You might try a forced restart and if that does not help a restore via iTunes. After that contact Apple Support for further guidance.
The microphone on my iPhone 6 won’t work