Based upon more than 500 posts in the thread running over on Motorola corporate, there is a range of response by the affected Atrix owners. Some report slightly muffled, while others cannot use the voice function at all.
In my own case, when I first bought my Atrix, voice transmission was clear as could be. Towards the end of the 30 day buyers remorse, I began to get requests to repeat myself. I didn't realize what was coming(fool I was!). Then, it got much worse to the point where I was yelling into the phone, and still had to repeat myself. It was embarrassing. I can no longer be heard at all....just a noise. Now, for calls, I use my little freebie Samsung flip phone on my personal line(the Atrix is mostly for business).
I would have sent back the Atrix to Motorola for a repair, but each Atrix owner who has posted about their repair experience for the muffled voice said that, the "repaired" phone was either the same, or worse. So, I don't see the point of being without device, only to get it back the same way as I sent it.
I've tried all the gimmicks and workarounds. No go.
I plan to replace the Atrix with a Nexus S on ATT bands, when Samsung finally releases it.