I had the same issue with my Lumia 820. I later found out that there is a mic at the rear camera assembly that was covered by my poorly-designed casing. It has 2 small holes that only exposed exactly the perimeter of the lens and flash. To test it, I turned on video mode and blew softly at the rear camera assembly (without casing). When I play back the video, the loud whooshing sound confirms that the secondary mic is near the camera assy. Using another casing that has a long opening, the problem disappeared.
I believe the Lumia 920 has a third mic also near the rear camera assembly (in addition to the primary mic at the bottom front and second mic near the earphone jack - as stated by some). That could explain why some users have the problem when talking with the phone propped up with their shoulders even without any casing, because the third mic is obstructed by the shoulder. For others, could it be due to the way the phone is held, for example, the palm covering the rear camera assembly?
If all else fails, it could just be a faulty unit. Send in for repair.