first delete the /etc/asound.state file then reboot. this forces slackware to read your devise and write a new one. then open a terminal and type alsamixer.
then press F5 turn up volumes and enable head phones set input to mic set capture and turn up the mic boost have fun.
the gui way. I guess click icon then settings then configure channels you want to use enable them and select input mic capture turn them up, turn up boost mic.
the other alsa oss mixer. Kmenu -> applications -> multimedia -> mixer select controls . enable select mic input select mic mic boost turn it up. if you have a head phone enable do it.
all three of those programs are linked to alsa. all three will do the same thing set one you set them all.
when upgrading from 13.0 to current you must read upgrade text
because there are packages that have to be removed.
is a good idea to read.
If it was a fresh install still read them.