had the same problem today. NOT a hardware problem, as I can get sounds form the phone. Seems some app or web site that I visited has put up a hijack on my sounds.
tried the vibrate and silent functions in all 20 possible permutations. RE-did the ring tone, rings when I enter it, but nothing will make the phone ring when I call it.
It vibrates (or not) when I use the sliders.
I cannot find any place to enable the sounds back to the phone, except the IMPLIED enabling with the silent button.
I also tried the silent button on teh side.
Nothing will make this little 4S ring out it's heart any more....