Fixing Low Sound on Linux Laptop Speaker

OS+Env: Slackware64-Current
Hardware: Lenovo Thinkpad T420i
Problem: Sound is working, but it isn’t loud enough
Solution: Create a file /etc/asound.conf and paste this config:

pcm.!default {
      type plug
      slave.pcm "softvol"
  }
  pcm.softvol {
      type softvol
      slave {
          pcm "dmix"
      }
      control {
          name "Pre-Amp"
          card 0
      }
      min_dB -5.0
      max_dB 20.0
      resolution 6
  }

Restart ALSA (/etc/rc.d/rc.alsa restart) and there should be a new mixer channel called pre-amp in the mixer software (kmix/alsamixer). And now, my laptop is ready to rock..

Ref: http://alien.slackbook.org/blog/adding-an-alsa-software-pre-amp-to-fix-low-sound-levels/