sound will only come on during charging

I keep losing all sound on my iphone 3gs the only way to get it back on is to turn it off and charge it for about 10 mins or so and leave it connected while i turn it back on and the sound will come back. but after a while when i disconect the phone it loses sound again. i have tried factory settings and and all the other tricks posted about to no help... it seems to lose all sound when i press the home button or plug the charger in again. anyone have any clue to whats happening?

Dave