I just picked up this CD in a battle on Shadow Moses Island. The item just dropped randomly in the middle of the fight. It happened to be the 5th Brawl I had played and it was also the one when I unlocked Ness. I'm not sure if this is just a totally random item or if it was dropped by some requirement but you will probably get it quickly if it's not random.
It's a remix of the original Super Mario Brothers theme music. It's so nostalgic to hear that tune again. This time it's been remixed into some kind of Jamaican reggae techno song. I have to say I like this edition a lot better than the original.
Every man dies, but few truly live. Live your life to its fullest, every day as if it were your last.