There should be 3 microphones, one facing forward (as you know), the other two facing rearward. The bottom one on the rear would be a dual-purpose mic, first when talking on the phone it serves as a noise cancelling microphone to source the surrounding background noise. The phone uses that noise as a reference and by comparing it with what comes through the front mic, it's able to cancel out some of the noise between the speech. There is also a third mic just below the flash at the top. If you look at the back of the phone, you'll see an elongated slot at the bottom of the Gray "mirrored" section on the top of the back side where the camera, flash and speaker are. This third mic is also used in noise cancelling, but is also a dual-purpose mic. In combination with the mic at the bottom, the two are the sound mics for video recording, and it will actually capture stereo sound by way of the dual mics.
Your audio recording options in Video are as follows:
Set sound recording options: Stereo, Wind Reduction, Concert, Balanced, or Front Facing. Balanced is a combination of the front facing and rear facing microphones so your voice can be recorded at a comparable level to those of the person(s) which you are taking video of.
