CD

From the Super Mario Wiki

A CD is a special item found in the Wii game, Super Smash Bros. Brawl. It may appear randomly througout any part of the game. Thus, it may appear in the Subspace Emissary, VS Mode, or any other game mode. When collected, it will allow players to add more music to their selection of My Music tracks, but the song contained in the CD is randomly determined. Other than this, the item has no other amount of significant value. The item can be comparable to the Trophy or the Stickers, as it is a collectible that is found randomly across the game. If Mew appears from a Poké Ball, it will drop a CD, but if the player has collected all the songs in the game, it will drop a sticker instead.

[edit] Trophy Information from Super Smash Bros. Brawl

Type	Image	Description
Classic		Each stage in Super Smash Bros. Brawl has several songs prepared for it. You can set the percentage chances for which songs appear in My Music. It's fun to choose songs according to your tastes. Each CD contains one song to increase your music library. Be sure to grab them when you see them. Once you've got all the music that's in the game, CDs will stop appearing.