High Definition Audio Device would be the integrated audio chip on your motherboard. NVidia HD Audio Device would be the audio over HDMI on your graphics card, and is reinstalled each time you update NVidia graphics drivers (although I think you can uncheck that box if you skip the "express" installation). These both could have been updated if Windows updated.I found something. I uninstalled all other audio drivers yesterday but they reinstalled themselves today. Maybe Sonar needs a component they supply. One was simply called High Definition Audio Device and the other was NVIDIA High Definition Audio Device. I never use either of them but it looks like the system is happier when they're there.