Go into Brawlbox and open up the brstm. Then select Export from the top under Edit and save the file, this'll get the wav file out. Pick any audio editor and import the wav song, I use Audacity, and mess around with the volume gain or sound volume on it.
Basically, put da volumez up.

Export the song as another wav again, and make a new brstm in Brawlbox with it using the same looping data as in the old one. And that's how you do it. If you make it too loud though it starts to bleed over the sfx.