no sound, no microphone on my iPhone 3GS

ok... my story, i have 2 iphones 3gs with a screen issue and the other one with sound issue. i bought them like that to fix them because i can't afford to pay more then $75 for a iphone still upgradeable by apple.

While i was changing the screen for no1 tried on the motherboard from no2 to see if i can get any sound... nothing, no speacker, no microfon, no headset so i put back on the original no1 motherboard and everithing works fine.

i put the no1 motherboard in the no2 phone and everithing is fine so ... i figure that is the motherboard who has the problem so i reset the phone to the original settings, blow air in the speakers and hedseat to clean them out, ... i dont know what else to do ... can you help me?

is the jailbreaking my only chance? i really want to avoid that