https://web.archive.org/web/20140819155058/http://vip.asus.com/forum/view.aspx?id=20080423032545000
This may be just valid for c-media based audio cards but.. who knows. I don't have the required equipment to test anyway.
EDIT: also this. Which microsoft after only almost a decade has acknowledged. EDIT2: there's this nice patcher available now
trivia: Windows also doesn't support DD EX