Aparently some guys developed a sonic mod for Halo custom edition. Here's the video And.... Here's what seems to be the homepage I have to say is a pretty nice mod, obiously some things need to be improoved (like the HUD) and they could increase the walking speed and jump rate to give a more sonic feeling to the mod.
I think they should go further then that with the conversion honestly, but what they do have is looking good. I'm particularly impressed by the Sonic models, as they seem to have managed to almost perfectly replicate the official ones from the Sonic Adventure games.
they probally ripped the models with the new technique that makes it possible to rip complete models out of 3D games . Cant wait till they completed this mod, or at least made it fun playable.