To quote the FAQ of a well-known audio player, "I have an ASUS Xonar card and am experiencing problems...
Asus Xonar drivers are known to be completely broken on multiple levels. Get rid of this card and get a card from another manufacturer."
I don't have personal experience with the card but I've read about a lot of people who have major problems with it. A lot of people with Windows 7 seem to be having issues.
Wish I could help you more.