Sonic Adventure 2 Cheats

Main Theme
To unlock the "Main Theme" (Live and Learn) in Sound Test, you must have the three stories finished (Hero, Dark and Last). Select 1P mode, Last story, and fight the Final Hazard again. After the Credits, go to Stage Select mode, choose any stage and mission and finish it. Finally, go to Sound Test and select "Character". "Main Theme" is above "Theme of Sonic".

Unlock Playable Characters
Follow the guidelines below to be able to play as those characters in versus mode:

Play as a Chao: Beat all of Tails' levels with an A rank
Play as Amy: Beat all of Sonic's levels with an A rank
Play as Big: Get A ranks on all of Eggman's Stage
Play as Chaos 0: Beat all of Rouge's levels with an A rank
Play as Mecha Sonic: Beat all of Shadow's levels with an A rank
Play as Tikal: Beat all of Knuckles' levels with an A rank

Trophies
There are 8 Bronze Trophies, 5 Silver Trophies, and 2 Gold Trophies.

Beyond Good And Evil (Silver): Clear LAST part in story mode.
Boss Attack (Bronze): Clear all 3 modes in Boss Attack Mode.
Chao Raiser (Bronze): Raise all 5 properties of any one Chao up to level 10.
Chao! (Bronze): Name your Chao for the first time.
DARK! (Bronze): Clear DARK side in story mode.
Emblem Collector (Silver): Unlock 90 emblems.