When I get a call I have to take it in speaker mode because I can't hear anything with the phone closed. When I open it and it's in speakerphone mode I can hear the call. I've tried cleaning the phone many times, doesn't help.
Have you recently used a headset or plugged in earphones? There were issues with the phone still thinking the headset was plugged in - try plugging it in again and removing it carefully - or perhaps a battery pull to reset it and it will go back to non-headset mode.