Most users of such Motorola ATRIXs claim something along these lines:
"The voice quality is such a problem that almost every person I talk to tells me that I either sound like I am in a hole, in a cave, far away from my phone, on hands free, muffled, or just plain your phone really sucks."
The good news is that Motorola is well aware of the problem and is searching for a way to sort out this issue. Given the fact that the Motorola ATRIX 4G is one really brilliant device (our review here), we hope this would happen sooner rather than later.
source: Motorola Support Forums via Engadget