I don’t think it has anything to do with Wavelab 6. Even with Wavelab 8, audio device selections seem to be about the same as they always were. Possibly the same with Wavelab 9. You could try the Wavelab 9 demo when it comes out, but I don’t think it’ll make any difference.
I think it’s more a Windows 7 thing, the removal of Stereo Mix, on some computers only, with some drivers, or possibly even different editions of Win 7 I’ve seen noted (Pro, Standard). But there are possibly workarounds:
http://answers.microsoft.com/en-us/windows/forum/windows_7-pictures/no-stereo-mix-in-windows-7-please-help/9bf67267-329a-45f5-a6ce-902c6fa6b4bc?auth=1