Many iPhone 5s, 6s, 7+ and iphone 8 with iOS11, users are facing issue with sound recording while shooting a video that video recording sound drops off suddenly at some time slot and come back again when playback. However if you try to record the video from front camera there is no sound recording issue with iPhone. The same issue was reported couple of year back with “iPhone 6 sound missing from a recorded video”.
When you record the video, the video looks good but the only sound is a hissing sound and the audio is very very low to the point where you can only hear it if you hold the phone directly to your ear, which is very annoying. What is more strange that the voice recorder still works well if tried standalone. Sometime there is no audio recorded at all with iPhone 7 video recording but sometimes recorded videos sound drops out to about 20% during playback. Resetting the iPhone does not seem to help the users. However at very first place you should check the case you are using to cover iPhone, note if a case that might cover the mic of iPhone and causing the isuse. But it is not the case than you should read further fro fix :
How to fix no sound when recording video on iphone 6/7/8
If you have noticed that there is a small black dot between camera and flash is a microphone, this is useful for video recording using the rear camera and for digitally eliminating background noise when you're on a phone or FaceTime call.(Source - Apple support.)
This is a micro mic in-between the camera flash and lenses so the clear protective wrap might have blocked the sound recording. Please remove the wrap over it after removing the wrap, the new videos taken should not have this issue anymore.