That whole "under" the home-keys (on the chin/lip) is the primary mic (i.e. the voice mic). The second mic is easily overlooked (as Seidio found out). If look at your phone on the back side, just below and to the right (towards center) of the mic plug there is a small.... indention (for lack of a better word). That is the input for the second mic. Many people (myself included) that complain about "echoing" in calls are covering this in some fashion (I was covering it, unknowningly with my finger before I got the case that slightly changed how I held the phone, NO more echo). As I mentioned, the early Seidio cases forgot to include this whole and the call quality was effected. A lot of people overlook it which is why I am/was concerned that these custom cases do not. I will not be happy if I have to dremel a $60 case.
You can see the location better on this image: