Not bad
You should check on it in flash player a couple of times before submitting. This I say cause it looked too fast.
You could've either lowered the frame-rate or put some double frames inside movieclips. The animation of S/Zonic standing looked too sped up and the background when they were running looked a bit weird.
Anyhow the sound fx sounded good and the idea of a green hedgehog was ok, although not too original.
Also the battle was a bit non consistent.