You dont, that utility only sets audio levels for the various channels.
If you want to do any kind of audio processing (for example, add echo, enhance bass) you need to connect /dev/dsp as input and output with sox as the audio processor in between.
You should be able to install sox using either
apt-get install sox
yum install sox
(don't know slackware very well anymore -- download from the site otherwise)
Read about it on the site, audio manipulation requires some knowledge