It was PITA to make the f NT 4.0 to have "ICH AC97" audio under VirtualBox*
No idea what helped but I did install the following:
* (post install M$ SP6a)
* vbox driver CD - make sure the "VBoxGuest Support Driver" is enabled on Boot not System
I have the feeling the last one is unnecessary, maybe too lazy to re-install that crap again :-}
* tested on OpenSUSE with Vbox 6.1.32 YMMV