One of my colleagues received a mail from Matthias with the question how to find the stereo mix device in his recording devices, with the previous versions of Vista these devices where visible. The next day he sent us a mail that he had found the solution. I just wanted to share the solution with you.
Open the Playback or Recording devices dialogue box (right click on audio icon in systray). Right click in the dialogue box and select show disabled devices, from here you can enable the devices and start using them.