John
Sonar can play back HD video. That is video that has a resolution of 720 i or p and/or 1080 i or p. HD audio is a term used by some without any meaning.
The concept of "HD Audio" isn't that much different. Anything that was recorded above 44.1 16bit could be considered HD audio as it by nature has a higher resolution that what was considered "Standard" in digital audio recording.
Video can still look terrible at 720P or 1080P (or i) and the same can hold true of audio recording above the standard.
(you also wouldn't define Audio Tape or Film in terms of resolution...we are talking about the digital realm here...so yes, HD audio is defined the same way, IMO).
As for Sonar using HD Video, it can play some but not all, and the conversion output is less than ideal, IMO.