Question about Camtasia audio recording?

Basically, I have Camtasia, and I want to do a let's play with minecraft. I want to record the video so that there is sound coming from my microphone, but because I don't have speakers, I want there to be sound coming from my stereo mix as well. That way the viewer can hear my voice, and the sounds of the game. While I can't find an option to do this in Camtasia, I was wondering if there was a way to make a "composite audio source" on my computer. That way I could have both audio sources running at the same time, kind of like superimposing. Is there any way, or any software out there that I could use to do this? Or will I need a hook up, outside of my computer?