When Apple decided to relocate the microphone on the iPad 2, it may have overlooked the fact that its sonic performance may have been affected by differences in design between models. On the 3G version of the tablet the mike is housed under a plastic strip, which covers the additional antenna, and not inside of the metal casing as it is on the Wi-Fi model. The difference in materials looks like a sensible reason for the “muffled” and “echo-prone” sound, and the effect is described as most noticeable when the iPad 2 is facing the user.
No comment on the issue has been given by Apple, but it seems like not that many users have brought it up yet. Nevertheless, with so many people expecting their iPad 2s to be delivered to them within the following weeks, Apple might have to act and tweak the tablet's design in the middle of its production.
source: iLounge via TUAW