Step 1: Why adding a mic is complicated
iPod compatible headphone with mic often have a button that can be used to play/ pause the music. the button is wired to the ground pin and the mic pin. From what i can find it appears that a diode is used in the process. My guess is that there is a current that constantly goes to the mic and ground pin. This current is AC power and when the button is pressed the current goes through the diode becoming one direction DC power. By obstructing the signal the ipod then detects the button is pressed and preprogramed responses are initiated. That's my theory anyways.