I have had the same problem and the only thing that has "fixed" it is the case. I bought an Incipio Feather case for my L928. After I received it and placed it on the phone, I was having problems hearing people. It was like the phone would cut in and out. So I thought, maybe it's something like the whole Apple-Gate where people "holding the phone wrong", so I got a call once and while I was experiencing the cutting in and out, I told my caller to hold a sec and I pulled the case off - voila!, problem solved. So I don't know if it's the small hole on the back above the flash (which I think is the light sensor) or the one on top next to the micro USB that is picking up reflective sound off of the case somehow. All I know is that when the case is on, I have intermittent problems (doesn't cause problems every call), but when the case is off - no problems.
So, I don't know how it's responsible...but it seems to be the culprit as I can confirm this.