Cheat Code

Metal Sheen:
     Hold  A + Y after selecting a 2-player level to have a "metallic"
skin covering your team

Unlockables

Super Hard Difficulty: Beat the game with 141 Emblems and all A ratings
Metallic Characters: Collect all 141 emblems
Last Story: Collect all 7 Chaos Emeralds and finish all stories 

Unlock Media

Team Chaotix Cinema: Complete Team Chaotix in Story Mode 
Team Dark Cinema: Complete Team Dark in Story Mode 
Team Rose Cinema: Complete Team Rose in Story Mode 
Team Sonic Cinema: Complete Team Sonic in Story Mode
Sonic Heroes Song: Unlocked at start
Follow Me Song: Complete Story Mode as Team Rose
This Machine Song: Complete Story Mode with Team Dark
We Can Song: Complete Story Mode with Team Sonic
What I'm Made Of Song: Complete the last story 

Unlock New 2P Modes

2P Action Race: Unlocked from start 
2P Team Battle: Collect 20 emblems
2P Special Stage: Collect 40 emblems 
2P Ring Race: Collect 60 emblems 
2P Bobsled Race: Collect 80 emblems
2P Quick Race: Collect 100 emblems 
2P Expert Race: Collect 120 emblems