My iPhone sounds aren't working why is this?

My iPhone sounds such as: Keyboard clicks, new message tones, lock sounds, new mail sounds, or any type of alerts aren't working, music also isn't audible. When I plug the headphones in I can hear everything, but without the headphone I can't hear anything except for the ringer. Why is this? Is there something wrong with the headphone port? or is there something wrong with the phone? I've had this phone since late February of 2008 and haven't experienced this problem until today. I have turned it off and on and that doesn't seem to help. Any idea what this is?