From the Super Mario Wiki
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“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. series
In Super Smash Bros. Brawl and Super Smash Bros. for Wii U, a CD may appear randomly throughout any part of the game, but stay a short time. When collected, CDs add a random song to their selection of My Music tracks. However, if a CPU controlled opponent picks one up, it will not be collected at the end of the match. In Super Smash Bros. Brawl, 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 in Super Smash Bros. Brawl and a Custom Part or a Trophy in Super Smash Bros. for Wii U.
Super Smash Bros. Brawl
Super Smash Bros. for Wii U