iPhone plus models loosing sound during video recording

I have had an iPhone 6s Plus and now have a 8 plus, whenever I do a video recording and playback the sound is intermittent, on my 6s I thought the phone was faulty so never had it looked at now I have the 8 plus and it is doing the same thing, anyone have any surgestions?

Ps: I changed the settings to record at 1080p at 60fps on both phones