At the moment there is no official driver for Windows 8 and the Windows 7 driver does not work very well. You may get sound, but the Audio Center and all the functions won’t work.
However, there is a 3rd party driver that works with all Asus Xonar cards.
You just have to download and install the Asus Xonar Unified Driver: http://brainbit.wordpress.com/2010/07/19/asus-xonar-unified-drivers/
I am using the version 1.54 and everything works perfectly.
I’m very glad someone bothered to write a proper driver. Too bad you can only donate through Paypal.