I don't think there is any setting you can change...
As far as a replacement goes, I guess it would depend on the severity of the issue, if it makes it so your phone has unacceptable quality on an unacceptable number of calls, then yes, get a replacement but be aware that the VZW is out of the Rezound and is replacing it with the Motorola Razr (not the Maxx, although a few users who have been through lots of phones claim to have gotten them) which is a vastly inferior device compared to the Rezound.
But to be honest, most Rezound users have this happen occasionally, not every time or in every circumstance, it happens to me once or twice a week depending on the circumstances and I usually just pull over or move to another room and the problem disappears. To me that is acceptable.
EDIT: LOL... I thought you said REPLACEMENT and just responded, I reread your post afterwards and it was asking about the mic PLACEMENT, my bad. I think the placement is fine, the implementation in the software is occasionally "skewd" but generally works decently. Don't get me wrong, 95% of the calls I make are acceptable, only under certain circumstances/conditions do I get a bad call, and with the state of the telecommunications industry, 95% seems acceptable in todays world, we are a long from the "five 9's" (99.999% quality and availability) that was the standard when I started working in telephony 20-some years ago.