Sound Test allows the player to listen to all the music and sounds in the game. Players can listen to the voices of characters, stage music, and listen to other variety of sounds. Sound Test can be accessed through Option Mode and Data Mode.
The sound test in all of the games has many unused sounds. An example is the original game, the announcer says "Bonus Stage", "Are you Ready", "Final Stage" and "Draw Game".
In Melee, the announcer has samples for "Master Hand", "Giga Koopa" plus "Giga Bowser" and "Fighting Wire Frames" and many more sound clips, these were most likely for him saying their name during the 1-Player games.
In Brawl, all of the characters have unused high-knockback sounds like the previous games, the fighters who did not get redone voicework have these reused in 3DS/Wii U.