Microphone only working on certain applications?

As I said in the question, my Iphone 6 microphone only works for some applications. For example, it works for my snapchat videos but doesn't work for the camera app. The apps that my microphone doesn't work on is the camera app and Oovoo, in the past these apps have worked perfectly and it has only recently done this in the last month or so.

I have tried dying the phone down till it has ran out of battery and charged it up, this seemed to work for a little while but then went back to it not working.

Can anyone suggest what to do?