Sonic/South Park

Version 1

 * Sonic as Stan Marsh
 * Rotor as Kyle Broflovski
 * Knuckles as Eric Cartman
 * Tails as Kenny McCormick
 * Charmy as Butters Stotch
 * Amy Rose as Wendy Testaburger
 * Vector as Chef
 * Uncle Chuck as Mr. Garrison
 * King Acorn as Mr. Mackey
 * Queen Acorn as Principal Victoria
 * Antoine as Randy Marsh
 * Sally Acorn as Sharon Marsh
 * Rouge as Shelly Marsh
 * Espio as Gerald Broflovski
 * Breezie as Sheila Broflovski
 * Chris Thorndyke as Ike Broflovski
 * Bunnie Rabbot as Liane Cartman
 * Ari as Stuart McCormick
 * Sonia as Carol McCormick
 * Mr. Tanaka as Stephen Stotch
 * Tikal as Linda Stotch
 * Hawk as Officer Barbrady
 * Emerl as Jimbo Kern
 * Trevor as Skeeter
 * Chip as Ned Gerblanksky
 * Cream as Bebe Stevens
 * Cosmo as Tammy Warner
 * Lucas as Craig Tucker
 * Rocket as Clyde Donovan
 * Big as Tweek Tweak
 * Captain Rescue as Token Black
 * Jet as Gregory of Yardale
 * McMole as Dr. Gouache
 * Elise as Nurse
 * Scratch as Jimmy Valmer
 * Grounder as Timmy Burch
 * E-102 Gamma as Pip Pirrip
 * Robotnik Jr. as Thomas
 * Silver as Bradley Biggle
 * Sleet as Bill Allen
 * Dingo as Fosse McDonald
 * Bocoe as Nathan
 * Decoe as Mimsy
 * Coconuts as Damien
 * Manic as Mr. Hankey
 * Athair as Jesus
 * Heavy & Bomb as Terrance & Phillip
 * Shadow as Scott Tenorman
 * Hacker as Jack Tenorman
 * Cheese as Towelie
 * Professor Gerald Robotnik as Scott the Dick
 * Snively as Saddam Hussein
 * Dr. Robotnik/Eggman as Satan

Explanations:

 * Knuckles is a in love with Vanilla
 * Shadow is a Knuckles' best friend
 * Sonic is a fat as his ego
 * Silver is a lot of accidents
 * Tails is a frequently scoloded.
 * Big is a chef.