When at the title screen, press A - 8 times, B - 4 times and RIGHT - 16 times on controller 2. Then start a game and when " Round 1 - Start" shows, press UP or DOWN on controller 1 to select stages. Beware though, a
lot of selections will most likely crash.
You'll have to complete the game and after the credits are over, press the B Button and an option for the sound test will appear.