I had the same problem as you (assuming you are trying to use the kX drivers).
From the reports it seems some people are able to get them to work and others aren't. I was one of the ones it apparently wasn't working for but after some investigation I found the problem that I was having. Most likely this is the same issue that others (and you) are having as well.
It seems the kX drivers change the default port used for audio out on your soundcard. With the way I had it hooked up sound plays fine with creative drivers in XP. However nothing played in vista with the kX drivers.
By changing the port in the back from audio out 1 to audio out 2, I suddenly got sound from the kX drivers.
The error you get on startup is related to the kX Sound Mixer not being supported by Vista. The sound drivers DO work. To get rid of this message simply download a copy of msconfig, open it up and go to the startup tab. Uncheck the box next to "kX-Mixer". Done, play your favorite song really loud in celebration.
One thing I did notice is that I get some weird clicking noises in the latest version of iTunes. This doesn't happen in WMP or Winamp. Sounds like an iTunes issue, but I'd be interested in finding out if others have this problem.