Sonic #1

Go to the Sonic 1 title screen and press Up, X, Down, X, Left, X, Right, Up, X, Down, X, Left, X, Right, X. Now HOLD B and press Start. Now when the game starts press A to go to debug mode

Pick Level & Sound Test

Go to the Sonic 1 title screen and press Up, Down, Left, Right. Now HOLD B to pick a level & get sound test

Sonic #2

Go to the Sonic 2 Sound Test screen and play sounds 19, 65, 09, 17. Now HOLD X and press START. Now on the title screen HOLD B and press START

Debug Mode

Go to the Sonic 2 Sound Test screen and play sounds 01, 09, 09, 02, 01, 01, 02, 04. Now at the sound test screen HOLD B and pick a level and Start

Be Super Sonic

After doing the Pick Level code above go to the Sound test screen and play sounds 04, 01, 02, 06. Now collect 50 rings and jump to be Super Sonic

Sonic #3

As you hear SEGA spoken BUT before the main menu appears press Up, Up, Down, Up, Up,Up. Then at the title screen press UP to get Stage select and Sound Test

Debug Mode

Go to the Stage Select screen and HOLD B and pick a level and start

Get Extra Stage

After doing the Pick Level code above go to the Sound test screen and play sounds 01, 03, 05, 07. Highlight the Special Stage 2 and HOLD B and press START