The guys over at iLounge, discovered the slightly microphone difference in the Wi-Fi and 3G models of iPad 2. The difference is about the quality of audio captured by the devices, Wi-Fi model of iPad 2 captures best sound quality where the 3G model of iPad 2 captures low quality of sound. The difference of quality between two models is caused due to the placement of microphone, in Wi-Fi models microphone is placed on aluminum surface and in 3G model is placed on black plastic, also the GSM and CDMA models shows different results.

iLounge Says:

Perhaps due to this change in materials, the Wi-Fi model offers markedly cleaner audio than that of the 3G, which sounds slightly muffled and echo-prone as a result. Curiously, we found the GSM 3G model’s audio to be slightly preferable to that of the CDMA model, which seemed to suffer the issues more severely.

This difference of quality does not affect you unless you are a big user of Facetime, or such applications who uses the microphone for recording all the time.