When you start slackware, before you do startx, type "alsamixer" and crank them all up to max and unmute (using the m button I think) all the columns.
They are muted if it has MM above the column.
After this, type "alsactl store". I had to be in root to do this, but it may just be me
But IIRC, one of the columns made my speakers make a static crackly sound, so if it does so, mute that column