Happened to me too, with my SB Live. Took forever to figure it out, and one of the problems was I couldn't figure out how to use alsa.
Check and post your modules.conf
Basically, switch the entries for sound-card-0 and sound-card-1 around so the sound-card-0 is your sb.
Assuming of course, you are running alsa
Other options include rmmod the onboard sound module, and modprobe up the sb module.
Cool