Like Dave, My understanding is that "High Definition Audio Device" is the Sound device used for HDMI. From what I understand, it is specific to AMD's Implementation; One of my systems is AMD-based and disabling the "High Definition Audio Device" doesn't affect audio output from the on-board sound, nor from the Xonar DG Card installed in it. It does prevent Audio from going through HDMI. Nvidia's version is "NVidia High Definition Audio" And Intel uses "Intel(r) Display Audio" which are far more clear in their naming.
The uninstalled "Unknown Device" would seem to be a sound card. Though Drivers don't tend to uninstall themselves, either.