i've seen these but don't know what they're calle, basically its a female jack port, a small box, size of a button, with a mute button and vol controll, then a piece of like 3 inch cable and a male jack, so you plug your earphones/headphones in this thing and then plug this thing into your mp3....
whatever you do, don't try and cut the original cable and attach volume controlls yourself, the wires are double insulated and are ultra hard to attach, dare i say, impossible for a normal person without the right tools ...