Thinking my Atrix was the best phone in the world, I ignored people who said I sounded "muffled".
I had this problem with my Motorola Milestone, and the fix turned out to be a slight hardware modification with some paper and glue.
Turns out it was a legitimate problem: I just called my Google Voice number to record the message greeting, and I could barely understand myself.
I removed the rubber grommet on the back casing of the phone, and the issue persisted. Turning off "Noise Suppression" helped immensely.
I have a fairly low voice (bari/bass) and it seems to me that Motorola hasn't exactly ironed all of the kinks out of its noise-canceling algorithms.
I can't fully blame them, but Apple seems to have gotten it right on the first try. I understand manufacturing problems can be a pain.
Maybe it's the price I pay for getting a $50 discount on a refurb unit! Someone probably already returned the unit for this issue, and QC isn't set up to test for it, in all likelihood.