iPhone 4 - Problem with sound when calling

Dear All,

My iPhone had LCD replaced and everything functions perfectly except for sound during call. I can't hear the other person and the other person can't hear me. It only happens during calls, I can record voice memos and play it both via both speakers (loud and earpiece).

What is more interesting - when i answer a call with the mute switch in off position iphone initiates loud mode and proximity sensor does not work. when i flip the mute switch to on iphone switches to earpiece and proximity sensor starts working, however in both situation there is no sound.

I guess there has to be a short circuit somewhere. I tore it apart and reassembled couple of times and managed to get sound, but proximity sensor did not work so took it apart got sensor working but lost sound again.

Thanks for your time and I appreciate your help!