A Sound Test is a feature allowing players to listen to audio at their leisure in a menu without having to visit certain stages or perform certain actions. This includes the option to hear entire tracks even if it is usually interrupted in a cutscene. It was initially intended so that sound engineers can test that audio plays as expected, since it became a relatively common element in video games along with a cutscene viewer option.
As the mode wasn't initially intended for player access in many cases, some earlier titles like Kirby's Dream Land made it hidden through cheat codes, but in more recent games they have ranged from available after starting the game as it is in Kirby: Nightmare in Dream Land to being unlockables by fulfilling in-game requirements. This may involve unlocking songs individually.
Select 3DS games like Super Smash Bros. for Nintendo 3DS, Sonic Generations and Fire Emblem Fates allow the player to fold the console while the game continues to play music through the headphone jack, saving some power & space much like an MP3 player.
Another notable case is E.X. Troopers which allows the player to press buttons while in the Sound Test to have a snare and drum sound occur, this is to simulate the experience of being a DJ. In addition, the sound test in the game continues to play and cycle through the unlocked songs while the player wanders around the 3D overworld.