Sonic the Hedgehog 2

• 1). Connect "Sonic the Hedgehog 2" to "Sonic and Knuckles" by inserting it into the cartridge-slot of "Sonic and Knuckles."
  
  
• 2). Enter the "Sound Test" mode from the options menu.
  
  
• 3). Play the following songs in this order: Song 1, Song 9, Song 9, Song 4, Song 1, Song 0, Song 1 and Song 8.
  
  
• 4). Select any level, hold the "A" button and press "Start" to enter the debug mode.
  
  

Sonic the Hedgehog 3

• 1). Connect "Sonic the Hedgehog 3" to "Sonic and Knuckles" by inserting it into the cartridge-slot of "Sonic and Knuckles."
  
  
• 2). Grab a vine in Angel Island to enter the "Sound Test" mode.
  
  
• 3). Press left three times, right three times and then up three times. If done correctly, you will hear a chime.
  
  
• 4). Grab a pulley in the Mushroom Hill level and press left three times, right three times and up three times. You should hear another chime.
  
  
• 5). Pause the level and press the "Start" button. Select a level, hold the "A" button and press "Start" to enter debug mode.