Thanks to Luigi Link for giving me this code....

European/American version:

If you're using a European/American version, change the country in the emulator to 'Japanese'.
 Then load the ROM and at the title screen press C,C,C,C,C,C,Up,Down,Left,Right.
 The game will go to the demo mode. Once in there, press Down+A+B+C, keep them pressed, and either wait for the demo to end or press start.
 If you don't release the buttons, you'll see some japanese characters with the names of part of the team, and then you'll get the PRESS START BUTTON words...


Japanese Sonic 1 (GM 00004049-01):

 Make sure that the country setting is set to Japanese. If it isn't, change the setting then restart the emulation.
 
 At the title screen, enter the code C+C+C+C+C+C+Up+Down+Down+Down+Left+Right. You will hear a ring. Wait for the Green Hill demo to appear. At the demo, press and hold Down+A+B+C. Keep holding this until the "Sonic Team Presents" screen appears. However, the game will load a different palette, and you'll see a secret credits screen (in Japanese). At the title screen, you'll see the Press Start Button.