Is your mp3 file audio volume too low? Using lame MP3 encoder using your Linux box, it is just a one command away. See command below.
We want an MP3 file to be 4 times louder then it was originally, below is the command.
lame --scale 4 opening-song.mp3 opening-song-louder.mp3
Of course you need to install lame. For Slackware, you can get it via SlackBuilds.org.
That’s it.