Sonic Advance Unlockable: Tails
Tails will follow you around during your missions with the help of this code. To do it, go to the character select screen. Highlight Sonic, then highlight Tails and press down. Next, highlight Knuckles and press L. Finally, highlight Amy and press R. Then go back to Sonic and press A. Now, Tails will tail you wherever you go in Sonic Advance.
Sonic Advance Hidden Songs
To get more than the 39 default songs in sound test mode, successfully complete The Moon Zone, Extra mode, or unlock the Super Sonic Ending. Three additional songs will be available.
Sonic Advance Unlockable: Moon Zone
Beat the game with all four characters and collect the seven Chaos Emeralds. Then, beat the final zone with Sonic. After you defeat Robotnik, Sonic will go to the Moon Zone as Super Sonic.