[John Grey2]

I had the same problem, searched everywhere for an answer with no luck. I took it to the Apple Store to troubleshoot, we only reset all settings (Settings>General>Reset>Reset All Settings) and the problem was fixed, when I put all my settings back to normal, it muted again so I reset all settings once again, and checked for the problem every time I changed a setting, and now the problem is fixed. Resetting all settings does not factory reset your phone, all your contacts, photos, apps ect. will still be there, only settings are reset.