Arduino Midi DJ Controller....Sound Card?


I am thinking about building a dj midi controller to work with Traktor Pro. I have made a good start and have a potentiometer setup and working within Tracktor (can control anything that can be controlled with a knob)

What I was wondering is, how hard is it to build a dj controller with a sound card so that I can listen to an incoming track? Can anyone suggest a sound card that would be suitable.

Many thanks.

Just incase anyone is looking for an answer to this topic I have managed to find a very cheap solution, I have got a usb external sound card from ebay for £1.80 this could be installed into the controller housing I intend on building. It just means the controller will take two usb slots as opposed to one.

This I found very useful when setting it up with traktor on Mac: