EliteBastards has an interesting article speculating about the future of Creative Labs. Creative was one of the first PC sound add-in boards (AdLib was the first that I recall). They are certainly the most successful of the PC audio card manufacturers.
The author talks about the threats to Creative caused by the changes to the Vista audio stack, increased competition in the discrete audio board market, and the dominance of motherboard audio. I think there are really two main factors which make life hard for Creative:
That's not to say that Creative is out of it. They still have a very powerful brand name and some nice hardware in their X-Fi cards. There are still reasons to own add-in cards. It's easier to get audio right on a discrete card than on the motherboard with all the signal noise right next to the analog audio traces. Creative is also the only real game in town for accelerated gaming audio and a lot of games still utilize it.
The article not only talks about the PC sound card market but also gives a decent overview of the changes Vista made to the sound pipeline.