Sound Test:
Are you past level 57? Yes. Good!
Save the game at any point and highlight that saved game. Then press B to return to the main screen. Hold Down + Start + B for about six seconds. As you continue through the game, more sounds become available.

Extra Power Punches:
Purchase two Power Punches at once, and use one. The remaining Power Punch will contain several strong punches.

Thanks to Revolution reader Conner Lewis!