Level select:

Press Up five times after the Aztec Adventure scroll closes on the
title screen. Press Right three times after Nino tosses money at
the other characters. Press Left, Down after all characters are
walking toward the left side of the screen. Nino's face will appear
next to the Aztec Adventure scroll to confirm correct code entry.
Use the D-pad to change the level numbers appearing below the face.

Sound test:

Press Start while playing the game. Press Left, then Right twenty times
after the faces appear. Nino's face will appear to the right of the
rainbow to confirm correct code entry. Use the D-pad to change the
numbers appearing above the face to select a sound, then press 1.
To resume game play, select sound 00 and press 2.