Missing outgoing audio on calls iPhone

I hope Im not posting a duplicate issue, but i havent seen this problem posted ---
My iPhone 3G has stopped working for outgoing audio on phone calls or voice recording with the microphone on the headset -- I have the original headset/mic that came with it as well as the 79.00 apple in ear headset -- everything works fine until a call comes in -- I answer - can hear the caller perfectly, but they cannot hear me -- First I thought it was the headset--but it does this with everyone I try and they all work on other iPhones --- the phone is no longer under apple care and the closest apple store is over 100 miles away --- there is a guy in town that replaces broken screens -- he said its the mother board and cant be repaired -- the phone works fine if I unplug the headset -- my audio gets through --- I havent tried it with blue tooth --
I am running the latest version of the OS -- and this didnt coincide with an update -- the phone has not been dropped or damaged and has not been exposed to water ---
Has anyone seen this before? any Ideas --- Not quite ready to go the the 4 yet --
Thanks
iPhone 3G iOS 4