I have a 754 pin Motherboard like yours. Microsoft Update gave me a driver for my sound and it works fine. I have Realtek AC97 sound. I'm not sure if I remember right but it seems like when I checked for it I had to check for optional updates. Did you check for optional updates or anything like that?
I looked on the ASUS and MSI sites for a driver for Vista and there was none. I'm confused as to why M$ would have a driver for W7 but there isn't one for Vista at MSI.
Maybe the one that "TechSupport" mentioned will work if you really can't get one at M$ Update..