The NM256AV is actually a video card controller. The NM3298 is your audio controller as you stated but there appears to be no sound card drivers online for it. Maybe you do a search for it since you have a better idea of what type of soundcard and system you have. I'm not sure which drivers would work with your soundcard.
*edit* Err... correction... I guess the NM256AV doubles up as an audio controller as well... heh like I said... it's hard to know when you don't have the system right in your hands
If this is a sony laptop there is a site that talks about setting up Linux on it: