You're gunna need to do better than this...
...especially for an 8mb flash movie.
Graphics: Work on them. Alot. I shouldn't be able to tell that the graphic was cut out of another image, and there are SEVERAL pieces of evidence of this not working - most notably Master Chief right at the beginning. Spare white pixels on a black background is simply too easy to notice, as is the not-above-par cutting job on the monsters later in the movie.
Sound: This is the only area you really did ok in - the music did fit, but it isn't worth 8mb. Theres not much for sfx, but there also isn't much of a place to put it.
Storyline: Dunno, its apparently an MMO in Flash. Two words: Copyright. Infringement. Microsoft went batsh*t over the Halo RTS project, so be extremely light-footed here. If they get wind of this and its looking to be a bad reflection on the Halo name, don't think they won't can it. And they might can it if it's a good reflection on it as well - they own Halo, and they want the profits from it.
Overall, I ended up ranking it a 0. You're looking off into the summer of 2008 and you're already releasing a trailer without much else going into this. I've worked with online games before - 5 to be exact.
0 have seen me change the build mode from Debug to Release, because 0 had the bugs worked out of multiplayer before release.
Get yourself a team if you really are serious in putting this together - even with 5 team members fluent in Flash you'll be hard pressed to get it to work properly. Me and three others couldn't get a c# game off the ground because we couldn't interact with the server properly - maybe you can do it with five.
And if this does happen to see the light of day, I apologize. But if you're looking out into the summer of 2008, you're looking at plenty of opportunity for disaster. Good luck, and I do hope this doesn't get blasted by Microsoft if it really does take off.