Why my microphone is not working when I recording video?

Then take a look at this support document, If the microphones on your iPhone aren't working - Apple Support

The last section states that if the app has permission to use the microphone and is not working, then contact the app developer. If the app has permission to access your microphone, or if it isn't listed, contact the app developer.

This happened to me to with my iphone 7plus. It happened after the new iOS update and won’t record Snapchat, Instagram, camera, FaceTime will fail and phone calls can’t hear me. It would work after restarting my phone but now even that stopped helping and I factory reset my phone as well.

Recording video or screen recording?

Internal audio is automatically muted while screen recording

Check Settings>Privacy>Microphone and see if those apps are listed there as getting permission to use the Microphone.