You can use Traktor with just this bad boy for $1 and the internal soundcard in your computer.
It's fairly easy combining audio sources on MAC cause it's built in to the core platform. On PC it's a bit different, but there is a software that will help you resolve this issue. It's called ASIO4ALL and is a freeware that is available for download. I'm not quite confident that the stability is 100% so if you wan't to use this option... Make sure that you've tried it out properly. Digitaldjtips explains the process Software is here