Here's a theme song list of my characters.
Only two more songs are missing.

Sonic:
Sonic: It doesn't matter (Tony Harnell)
Super: His world (Zebrahead)
Fighter: What I'm made of (Crush40)
Hyper: Live & Learn (Crush40)
Warrior: Open your heart (Johnny Gioeli)

Tails:
Tails: Believe in myself (Karen Brake)
Super: Sonic&Knuckles Super theme
Angel: Divinity II (FF7 AC (Nobuo Uematsu))
Spirit: Chaos Angel (Auron59)

Knuckles:
Knuckles: Unknown from M.E. (Marlon Saunders)
Super:
Vampire: Mephiles Phase II (Sonic Next Gen)
Hyper: To teh Max (ChEsDeRmAn)
Nosferatu: Symphony of Pain (Grumbleduke)

Shadow:
Shadow: I am... all of me (Crush40)
Super: All hail Shadow (Magna-Fi)
Reaper: Those chosen by the planet (FF7 (Nobuo Uematsu))
Hyper: Jenova Absolute (FF7 (Nobuo Uematsu))
Devil Reaper (good): The chosen one (A2)
Devil Reaper (evil): Figure.09 (Linkin Park)

ShadowNCS:
NCS: Who I am (Magna-Fi)
[comment: It might be a bit of a bad choice using this as the theme song of NCS, but I can kinda imagine how NCS is yelling these phrases at Dr. Ivo Robotnik (Eggman)... And it's hard finding a vocal theme song for a fan character]
Super: Crisis City (Sonic Next Gen)
Magician: Meltdown (DavidOrr)
Hyper: Outer Space (ParagonX9)
Sorcerer (good): Requiem (Wahnsinn)
Sorcerer (evil): Wake up (make a move) (Lostprophets)

Nightmare:
Nightmare: Never turn back (Crush40)
[comment: After I've listened to this song a lot of times, I decided to make this to Nightmare's theme song.
Verse 1: Part 1: Nightmare is talking about how he felt when he first came to Genloyas (NCS' dimension).
Verse 1: Part 2: He's singing how he feels his life as the guardian of time and space.
Chorus: Part 1: He's singing, that he is arrived at a new place (The place of Sonic and the others)
Chorus: Part 2: He's singing about the last event of chapter 10 (It's planned to be at the end of chapter 10 at least), which he wants to undo, but he realizes, that he better shall "never turn back".
Verse 2: He's singing, why it is be better, that he doesn't look back.]
Super:
Mirror: Black Nu-VikingMetal (Wahnsinn)
Hyper: Zero Gravity (ParagonX9)
Reflector (good): Dancing with the Spirits (DavidOrr)
Reflector (evil): Shut your mouth (Pain)

Eggman:
Eggman: E.G.G.M.A.N. (Paul Shortino)

To see the characters in their different forms, click here.