Begin a two-player game, then, at the Game Time menu, hold Up and B on Controller 2. Now press A or Start on Controller 1 and the cheat should be enabled.
At title screen, after "Push Start" appears, hold the A button and press Right, Up, Left, Down, Right, Down, Left & Up. Now, release the A button, hold the B button and press Right, Up, Left, Down, Right, Down, Left & Up. If done right, the sound test will appear.