Ok so I found a quick simi-solution to this problem for people with non-jail broken iPhone 4s's and it also doesn't void your warranty.
The problem is def due to a bad docking port in the phone, and if your like me, it wasn't from water or bad use of the phone. It has to be a manufacture mistake or cheap parts. What I did was take the back off your phone. Real easy, two screws at the bottom then slide back upwards. Once you do that. You can test to see if this will fix your problem by pushing the dock connector down with your finger in the bottom left hand corner. If this brings your sound back then you should be able to do what I did. I folded a tiny corner of a sheet of paper twice and put it on that corner of the dock connector. Then put the back back on and the screws back in. The paper puts enough pressure on the dock connected to fix the problem. If this works for you, please let me know my email is nick(at)thismegabites(dot)com if you aren't worried about voiding your warranty I do suggest just replacing the whole dock connector in your phone. It's really not too difficult, but I didn't need to for this worked well for me.