You need to separately do a manual install of the Realtek audio driver in addition to the MS driver, get the latest from ASUS: http://support.asus.com/Download.aspx?SLanguage=en&m=Maximus+IV+Extreme-Z&p=1&s=39
(this is the win 7 64 bit)
That driver fixed both my HD sound and the mic.
i did a manual install of the realtek drivers tons of times. they are working nicely. what i cannot find is the microsoft drivers. i cannot even find them in my windows update list so i can uninstall them. but like i said my sound is working fine. and i have 2 gtx 580s with the sound driver for them. maybe thats why im still getting sound. if anyone can tell me where to go to reinstall the microsoft driver that would be great.