During gameplay, pause the game and press B, A, A, Up, Down, Down, B & Right. If done right, a sound will be heard.
On the title screen while 'push start' is flashing, press the following combination for a sound test. (Then select the tunes with A or B, play them with 'Start'.)
Anwhere during a stage (preferably at the beginning of the game), pause the game, input the following combination and unpause again. A jingle will confirm success.
(Don't lose a life, this can only be used once.)