Your audio card has a mixer in it (it can also be called "Stereo Mix", or possibly "Wave Out", "Sum", "What U Hear" or "Loopback" or something similar). If you go into sound properties, there should be a volume slider for it. I don't remember if it's in the recording or the output sound setting. It can be pretty nifty feature for recording (let's say you are recording yourself playing a video game, it would record input from the mike as well as output from the game). One of my old desktops had that feature, but my current one doesn't; I wish it did.