I recently came across these Nokia pop-port connectors that provide a 3,5mm headphone output jack for connecting the mobile phone (=mp3 player inside) to any stereo system. As I wasn't willing to pay all these crazy shipping charges on ebay, I was triggered to build this thing myself and consequently only bought a cheap hands-free headset for something like 1,- EUR...

Inside this 4mm cable are just three simple microphone wires that connect to the 3,5mm output jack and on the pop-port connector from that cheap headset to three different pins.