In Nuendo 4.3 you need to go to Devices then click on Device setup. That’s where you choose what audio device (sound card) you want to use. Then you have to go back to Devices and click on VST connections and setup your audio device (sound card) in there also.
Also just to be sure, go to the control panel on your computer and then click on Sound to make sure your computer is reading the same sound card as Nuendo 4.3 - Good luck!!