I have always been a tech geek from way back. Love the stuff!
iPhone 3GS Battery
It sound like you either have a cable not seated correctly, did not replace a screw in the proper place, or possibly have a defective screen. If the metal covering on the microphone is not in place correctly it could also be shorting out on something. I would carefully look at everything touched in the process and make sure it is correct or worst case remove it all and start over. Look at the pictures and see what looks amiss on your phone. Without actually seeing the phone it is hard with these types of things.