Iphone 4 volume not working on apps but works for videos?

Recently my iphone 4 running on 5.1.1 volume stopped working only works for videos but when doesn't work for incoming calls or apps. Anyone know how to fix this problem? I tried a hardware reset still no luck.

Go into settings. General. Reset. Reset all setting and content. Wait for it to finish. Plug iPhone in to iTunes. Done it's fixed. Back it up first

Is the little switch above the volume control have an orange dot? If it does switch it the other way and it should work?