The Sound Player is a collectible item in Kirby & The Amazing Mirror and Kirby: Squeak Squad, and must be found before the player can access the Sound Test menu. In other games, the sound test is built into the menu system at the start, so the Sound Player is not present.
Some games have otherwise unused tracks that can only be heard with this feature. For example, Kirby 64: The Crystal Shards has a remix of Kine's theme from Kirby's Dream Land 3 that is not used, but it can be heard as Track 2 on the Sound Check.
You can listen to music and sounds from the game here—your list of sounds will grow as you clear games. Tap ←→ or use [the +Control Pad] to select songs and sounds. Tap the panel in the middle or press A to play.”
In the same game, there is also an unused sound originally meant to be played during a level in Shiver Star when Kirby stands in front of the TVs. This sound (number 398) is a reading of the Japanese poem, "Iroha", at triple speed.
When playing the Jukebox in Kirby: Triple Deluxe, red or blue music notes are emitted from the speakers. Tracks with red notes were composed by Jun Ishikawa. Tracks with blue notes were composed by Hirokazu Ando. This also seems to be the case with Kirby: Planet Robobot.
When playing a song on the Jukebox in Kirby: Planet Robobot, there is a rare chance that the Kirby wearing the headphones will change into an 8-bit Kirby, which will start dancing while the music is playing. He will stay until leaving the Jukebox.