From the Super Mario Wiki
“You guys know what CDs are right? That's right, those little shiny record-things! I still listen to records, but maybe you kids don't know anything but CDs these days... Whatever!!”
Wario Land 4
In Wario Land 4, each level (excluding the Hall of Hieroglyphs and the Golden Passage) includes a hidden CD that Wario can find. They are found in a CD Box (refer to picture on left). Unlike the Jewel Pieces and the Keyzer, they are not required to complete the game. Each CD holds a musical tune that can be played in the Sound Room, and the CD matches the color of the corresponding Passage it was found in.
Super Smash Bros. Brawl
In Super Smash Bros. Brawl, a CD may appear randomly throughout any part of the game.Thus, it may appear in the Subspace Emissary, VS Mode, or any other game mode. When collected, CDs 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. However, if a CPU controlled opponent picks one up, it will not be collected at the end of the match. 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.
Trophy information from Super Smash Bros. Brawl