Update:

There is a better method to make your Asus Xonar soundcard work on Windows 8. You just have to use a 3rd party driver. See this post for further information: http://black-pixel.net/asus-xonar-d2-on-windows-8.html

This is a tutorial on how to make your Asus Xonar D2 sound card work on Windows 8 Release Preview.

First of all download the driver at the Asus homepage.

Next you have to launch the setup in Windows 7 compatibility mode.

After finishing the setup, run regedit and search for [HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlClass{4d36e96c-e325-11ce-bfc1-08002be10318}0007Settings]

On your system it may also be 0008, you’ll have to find the right one yourself.

Change the SpeakerConfig key to 06 00 00 00 and the SpDigitalOut key to 01 00 00 00.

Now reboot and everything should work.

I found this solution at https://vip.asus.com/forum/view.aspx?board_id=21&model=Xonar%20D2X&id=20120301055430259&page=1&SLanguage=en-us