How would I go about lowering it?
Use BrawlBox to export it as a wav, change the sample rate with Audacity/Goldwave, then create a new brstm (BrawlBox will tell you what the loop point on the original is) using the fixed wav.
And I still would like an answer to the volume question.
Follow the steps I just gave you, but instead of changing the sample rate, increase the volume.
Note that if you replace a Melee song (any brstm whose filename starts with "W"), the song will be much quieter in the game.