Camera doesn’t record sound

I have Iphone 7.

Recently I have noticed that my camera doesn’t record sound for certain period while making a video. In other words, it records for example for a few seconds, then stops (there isn’t any sound on video), then records again. It is only in “camera” app. All other apps records sound very well, including voice memos, instagram, audio messages. I don’t know why and how to fix this problem, maybe some of you can help me ?