In my case it was also a mechanical problem. Wiggling the credit card in the dock connector slot did bring the sound back momentarily so I knew the connector had some bad contacts there. So I took some WD-40 and sprayed it in the connector and it started working all by itself! I also used the toothbrush on the connector afterwards to make sure all oxidation was removed.
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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.
In my case it was also a mechanical problem. Wiggling the credit card in the dock connector slot did bring the sound back momentarily so I knew the connector had some bad contacts there. So I took some WD-40 and sprayed it in the connector and it started working all by itself! I also used the toothbrush on the connector afterwards to make sure all oxidation was removed.