volume problems with 4S

I have a recurring problem with the volume of my iPhone 4S. The volume will randomly change from sound to vibrate with no pattern that I have been able to notice. Also, the volume button in the settings will go all the way down to zero without me having done a thing. I have restored the phone several times and even got apple to give me a new phone but the problem persists. I used to have the same problem with my 3GS also. Anyone else with these problems?