Level Select
At the main menu, go to options and highlight the 'Sound Test'. Play the following sounds 19, 65, 9, 17, then press start to go back to main menu. Next, when sonic and tails show up, hold A and press start for a level select.
100% success rate.
Super Sonic
After you have done the level select cheat, there is a sound test in the level select menu. Listen to the sounds 4, 1, 2, 6, in that order, then choose a level. When you have 50 rings or more jump up and you will turn into Super Sonic.
Submitted by Nasher. 100% success rate.