For you Magic Lantern junkies, here's a DIY video from Vimeo member Moritz Graeser on using the USB port to monitor audio from the camera. Of course if you're already using an external LCD monitor with a headphone jack, disregard. The instructions aren't super clear, but from a USB connector, you'll probably be using the middle two wires. Green cable being 'data -' and the white cable being 'data+'. The outer wires normally red and black are for Power.
On another note, shouldn't a basic Mini USB to 3.5mm Adapter for 0.99 cents + Free Shipping work too? Maybe i'm missing something though..
Mini USB to 3.5mm HeadPhone Jack