Hmm... the story isn't making sense
-Graphics- 7/10
The sprites are sort of okay, but really needs to be cleaned up and also Eggman's super thing with all the glowing yellow needs to be sprited, otherwise the graphics isn't that much of a problem. Also before using flash, script the text and make sure there are no spelling errors, I'm sure I saw a few there.
-Sound- 3/10
Sorry but you need to use the music more effectively, it's a bit too loud for a start, and as you stated, no character FX's when they move or anything, you need to work on this but otherwise, not bad. It was fairly good, you may want to use different tunes at different times too.
-Content- 4/10
There wasn't much content, even if you do state, you may also want to include an prologue so we know why Sonic fell from the sky, you need to make the plot more improved so we know what is going on, we want to know, how did tails know Sonic fell from the sky, how they managed to find eggman, also there is always dialogue between Sonic and Eggman before the fight, or nearly all the time, try and make a storyline with more depth and more detail, not cutting from 1 thing to another.
-Overall-5/10
Not bad at all, ok graphics, needs more sound and needs a better storyline and content, watch a movie and see how one is constructed to keep people coming back for more, there must be clues that link in to how people knows this and that, also how they fight and anything else, but overall for your first flash, i am quite impressed. Well done.