Sonic #2

Pick level
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