First, do this:

Log in as root in text mode:

alsamixer

Turn up Master and PCM, and make sure you have sound.

alsactl store

And next, go too http://www.slackware.com

RTFM, it tells a lot about audio problems like this, and you HAVE too read it. You're not going to learn anything if someone does this for you. A hint:

Look for where it talks about chmod on the *dsp whic is in the FAQ.