In my case it was also a mechanical problem and nothing related to software. I wiggled a credit card in the dock connector and the sound came back momentarily. So I knew that the dock connector on my iPhone 4S had some bad contacts in there. The toothbrush alone didn't help at all but when I sprayed some WD-40 in the connector the sound came back immediately all by itself. Amazing. I keep my iPhone very clean so it wasn't dirt. It must have been simple oxidation. So take some contact spray like WD-40 and give it a try.