No sound on iPhone 3GS

I know people have had this problem. I have no sound on my iPhone. It is like it is on silent. The sound works fine if my headphones are in, but other than that, there is nothing. The sound keeps coming and going in starts and spurts which is annoying. After reading other posts, I have a feeling that this is a common problem, caused by the iPhone thinking that the headphones are connected. Can someone please tell me what they ended up doing to fix the problem and where they had to go? I have visited a Genius Bar before but because I live in Canberra, it is a pain to go to Sydney for what could be a simple thing to fix.