Question [Q] microphone + ear speaker not working when i make a call

I have a LG Optimus black P970. My earspeaker and mirophone are not working if I make a call. On a call I don't hear the other person, and the other person don't hear anything as well... But I can still record with memo my voice and play it again. There everything seems to work. I have also a 2nd same kind phone and changed the microphone with the ear speaker with no change. Therefore I presume that it isn't a hardware fault. My device has stock rom Android Gingerbread with stock Kernel and is not rooted. So I made a upgrade to Android ice cream sandwich through the LG mobile support update tool without any change on the fault. Can someone help me what else I could try. I bought this phone for 80 and don't want to spent more money for repairs.