So, I've been trying to get Stereo Mix to work the past few days to record my system's sound and my microphone's. I've noticed that, if I have my speakers set as the default playback device, then Stereo Mix works, but if I set my headphones as the default playback device, it doesn't work. My headphones (Plantronics Gamecom 777) connect via a USB. It also has it's own sound card built in, so I figure that may also be the problem. I have Realtek High Definition Audio drivers.