While hanging, press 'Left', 'Left', 'Left', 'Right', 'Right', 'Right', 'Up', 'Up' and 'Up' - you'll hear a chime if done correctly, and then pause the game and press start, which will reset.
When the title screen appears, hold down 'A' and press start to bring up the level select.
You'll see selections for Sonic 3 levels, but these won't be selectable unless the game is combined.
While hanging, press 'Left', 'Left', 'Left', 'Right', 'Right', 'Right', 'Up', 'Up' and 'Up' - you'll hear a chime if done correctly, and then pause the game and press start, which will reset.
When the title screen appears, press 'Up' or 'Down' to select the "sound test" and press start to bring up the level select.
Within the level select, pressing 'C' will change the small numbers, to correspond to your preffered in-game character:
00 = Sonic & Tails.
01 = Sonic.
02 = Tails.
03 = Knuckles.
During the game, press pause, then press 'B' for slow motion, or 'C' to move to the next frame.