By default, swaret excludes the kernel from being upgraded. AFAIK if you're going to use the slackware alsa packages you need to use the kernel they were compiled with (ie. the matching slackware kernel in -current). You will either need to update your kernel, or if you have compiled from source then you probably need to compile the alsa drivers from source with the new kernel. Following the steps for your sound card at
http://www.alsa-project.org/ will solve 99% of sound issues.