Right, Right, Right, Up, Up, Up you should here a chime. press start then A it should reset when the menu appears. Press up, select the new option "Sound Test"
C=Reverse Gravity B=Enter/Exit Debug
(in debug)A=Change Item B= change back C=Place Item
Press start and when No Way! appears, press A,B and C together and you'll be in a new bonus stage. (Note down the level code also)
If you dust off your old copy of Sonic 1 and put that in S&K and do the same again, you have an almost never-ending series of bonus levels, each one different from the last, and if you entered the codes you got from your other, non- Sonic games, you can play those levels again!!!