Just found this. Check it out, looks decent
Step 1: The Parts
You will need:
-.1 micro farad capacitor
-220 micro farad capacitor
-10 micro farad capacitor --only necessary for the 200k boost loop
-LM386 microchip (or whatever it's called)
-small circuit board
-speaker --http://www.josepino.com/circuits/index.php?howto-speaker.jpc--make your own
Note: I added a breadboard that I drew up from the circuit diagram, so if someone wanted to double-check it that would be nice
Step 2: Liberating the audio jack
Strip the wire
Strip the insulated wires, making sure that you don't strip them so far that the stripped part touches the non-insulated wire
EDIT: Don't twist the two (formerly) insulated wires together, it might hurt sound quality.
Those (formerly) insulated wires are the positive and the noninsulated are the ground.
Check out my instructable on headphone repair for more about headphone wires.
Step 3: Place everything on the board
Here's how I set up mine