disclaimer: I'm truly shooting in the dark here. Consider the following post as brainstorming.
How many soundcards do you have? Is there an onboard along with another (perhaps) PCI soundcard? If so, and if it's the PCI one that you wish to use, then try the bios and make sure the onboard one is disabled. I think a list of cards recognized on your computer can be found in /proc/asound/cards.
Also, is libao installed? Perhaps you have some conflict between arts, esd, alsa, and/or oss.