I have the same card ..and the same problem
I ended up having to use alsa-driver from the kernel (2.6+) instead of the slackware package and then change sound support to be loaded as a module ( soundcore.o). In a nutshell, I had to recompile my kernel with everything I needed for sound enabled as a module instead of hard coded. Then of course, 'alsaconf' and 'alsamixer'. I have no idea why, but that's what fixed it.
regards,
...aaron