Here is a simple project where you turn an old house phone into an Ipod headphone. From there the possibilities are endless. I sometimes try and fool my friends into thinking that I am actually talking on the phone when in fact I am listening to my music. Note: The microphone will not be recognized by the ipod. I used some soldering in this project due to the fact that I broke the wires to the speaker and had to reconnect them.
Step 1: Why adding a mic is complicated
When I originally made this intractable the concept of how an iPod mic works was beyond me. I am inserting this step to better explain why i could not simply ad the mic. iPod headphones that have microphones sometimes have a button as well. Normal headphone jacks have three pins. A mic headphone jack has four pins. I have not found anywhere to just buy a four pin 2.5mm jack. The only way i know of buying one is the headphones that have the mics already attached. Griffin Technology does sell microphone adapters for headphone in which you can use it when wiring up the headset.
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.