This is mentioned all the time in the Slackware forum just to let you know but follow these steps and it should work.
Code:
$ su - root
(password for root)
# vi /etc/rc.d/rc.modules
(scroll down to the line that says #ES1370 Support for Sound Blaster PCI128:
and uncomment the line next to the # mark that says:
/sbin/modprobe es1370
by highlighting the # mark at the beginning of the line and hitting the x key.
Now type :wq! and that will save the file so it loads the module for your card everytime you boot into linux.)
# chmod 666 /dev/dsp*
# chmod 666 /dev/mixer
# shutdown -r now ( to reboot )
And upon bootup, it should detect your card hopefully and load the module, and sound should be working.. its as simple as that.