Graphic drivers from nvidia have been checked - they are showing as the latest.
On Windows Update? Or did you go to nvidia.com and download version 185.85? (or use their update widget thing)

Also, make sure you disable Aero.

The thing that matters most in the WoW config is your multisampling setting, in the resolution tab of the video options. If that's not set at a 1x multisampling setting, you're going to get a significant performance hit. So I'd maybe double check that, since it's not usually the most obvious thing in the wow video options.

As far as sound from all 5 instances, that sounds like you have "Make game believe it is foreground window" checked in ISBoxer -- I'd uncheck that.