The first of the two microphones inside the iPhone (the one that you speak into) is the Knowles S1950.

Knowles describes its MEMS microphones as being built with a patented CMOS/MEMS technology that allows for very small footprints, lower profiles, and several mounting options.

Translation: This microphone is very small -- a 3mm x 4mm footprint! And it's surface-mount, which means that a machine just pops it onto the logic board for you.

From a manufacturing viewpoint, a surface-mount design is valuable because it allows the components to be integrated to the logic board all at once. This eliminates sub-assembly costs, and lowers the BOM cost. It also allows us to have components on both sides of the board, something that's not possible with thru-hole technology.