Yeah - I didn't mean that it would/should fix the issue, only that Windows itself will release the drivers.  My explanation was in response to another poster's comment about disabling it in Device Manager.  Nothing so for explains what is happening with Sonar.
 
I have to help someone remotely with some computer issues for a couple of hours, then will circle back to your issue and see if there is anything I can do to help you get this figured out.
 
Bob Bone
 

I have a Roland V-Studio 700R interface (similar to the Octa-Capture) and I've found that, after a X3 crash, the SONARPRD.EXE process takes about 5 minutes to shutdown on its own after turning off the audio interface. A full reboot is rarely necessary. I recently mentioned this behavior to a Cakewalk support rep and he said that it was the first that he had ever heard of it.
 
-e.B

I know you said that you had the latest drivers, but Roland also has an updated Firmware file for the OctaCapture which solved quite a few issues I had. This latest firmware is required for the OctaCapture to work correctly in Windows 8, if you are ever thinking of upgrading it will be a required update anyway so you may as well update to save any issues in the future. It also adds the function to auto detect your Sampling Rate in Sonar, so you don't have to disconnect and reconnect to change sample rates in different projects.

...wickedOther media apps and browsers will still play sound though...

using the same asio driver?
 

I have an octa-capture and i notice too that the sonarprd.exe process takes forever to end after Sonar crashes.
 
I do not have the patience to wait 5 minutes so I have always re-booted.  
 
Since they fixed the enable-midi out bug, I have not crashed for a long time (yeah!).  But while that bug was alive, I experienced the behavior described with sonarprd.exe hanging forever.
 
JR