I had the same problem with Sonar not seeing my Studio Capture driver.
It started when I forgot to turn on the unit before I opened Sonar.
Somehow the driver got corrupted. When I re-installed the Studio Capture driver, the problem went away.
 
 

@soundtweaker:
 
Thank you for your suggestion, but I've reinstalled the Octa-Capture driver twice now, and the problem remains. And now the unseen audio interface problem is back with a vengeance...Sonar refuses to see the OC at all. I can't play back a project, and I can't record anything.

I believe the Melodyne problem is now solved, thanks to Cakewalk Tech Support.
 
The theory is that Melodyne had become corrupted, which I apparently fixed by reinstalling the program. The technician had me start a new project and use Melodyne, which worked correctly, so this seems plausible. However, my existing projects were still referencing the corrupted .dll file.
 
So we opened an existing project in safe mode, disabling the Melodyne module, and then did a manual scan of my VST folders. This ended the reference to the corrupted module, and Melodyne then functioned correctly within the existing project.
 
My previously existing Melodyne changes are all gone (at least in the one project I've checked), so I will have to recreate those...a hassle, but much preferable to the alternative.
 
As for the Octa-Capture,it started working properly about 45 minutes before I got on the phone with Cakewalk. Go figure.
 
Thanks to everyone who volunteered suggestions. Much appreciated.