First you need to go to the "Touch Start" screen, press Up(2), Down(2), Left, Right, Left, Right, B, A to view a hidden screen.
New Game + Mode.
You must successfully complete the game. Then, select the arrow next to the "Plus" icon at the saved game selection screen to start New Game + mode. In this mode, you will retain all level stats and items from your first playthrough.
Successfully complete New Game + mode to unlock the "Sound Test" option.