Good atmosphere, but...
Story: the story is nice, good introduction to it. However, the animation itself really didn't lend itself to well for it. It has an anticlimactic end. Fortunately, it wasn't one of those things where the character happens to have a fire arm or something to fight a monster. Still, having the creature run off after one hard hit isn't much. Perhaps more backstory on what the creature may have been. It could also help if there was more to the reason why it ran, such as a noise or if the guy was an athlete. I'm thinking the latter, but not too sure due to the lack of story detail.
Atmosphere: The atmosphere you managed to create works well. The audio selection and placement went great and truly made this animation worth looking at. It's dark and the viewer can get a sense of how the guy must feel.
Animation: It's a very simple animation, but it does have some nice details. You have a great drawing style. Hell, your alot better at drawing ears than I am. Some of the subtle movement, like the eyes going back and forth, were also well done. Another plus is that the gradiants weren't overused, a problem that many new Flash artists tend to have. Your shading can use some work, but it's not as bad as most people out there. The color selection also works well. About the only problem I see is that the movement is simplistic. Half the time, it looked more like paper doll limbs rather than actual movement. I think after some time, you'll get the movement down well.
Overall: Your definitely on the right track. You know how to set the atmosphere, you don't make any noob mistakes, and I enjoyed it despite the ending. You have great potential that can be reached through improvement.
On a side note, you shouldn't listen to anyone that say you should change in accords to their prefered style. I see a couple reviewers who have done that. Just work and improve in the style (or styles) that you like most. If you keep changing to meet the standards of every critic, you'll find your work becoming bland. Don't ever allow your work to become bland. Keep what you have now and just improve where it is needed.