Sound issues when recording videos

I have very much enjoyed my iP7+ since Jan 2017, however I have recently encountered a frustrating problem. When recording videos utilizing my rear facing camera, the sound starts recording fine but nearly always drops to about 50%. I’ve tried recording using the front camera and I don't have this issue. Ive force restarted and that did not help either. Not saying that is the culprit, but it seems to have started when i updated to iOS11 a few weeks ago. I’ve recorded many music performance videos over the life of the phone and have also had the same phone case. The rear microphone isn’t being blocked. Any help would be greatly appreciated