My iPhone video recordings have very low sound, can't turn it up

My iPhone 5s records videos with extremely low sound. I'm not able to turn up the volume at all, and I've tried to reset everything and it wouldn t work. On any other app that uses sound recording, like Snapchat, it's loud and works fine. I can hear music, watch movies, etc perfectly its ONLY this app. Can anyone help please?

It's way the camera was designed for that phone unless you have brightness turned down.