Lotta potential
This isn't bad. I've definitely seen worse flashes. Obviously everyone's commented on the art, so I won't go there. My suggestion would be either to hire some voice actors or just use in game sounds. I mean, usually one guy doing 90% of the voices doesn't work unless you're a HIGHLY talented voice actor with lots of experience, or you're the creators of South Park.
And for the record, the writing is pretty good. Some of the visual jokes were funny (like the sudden shot of the demoman passed out on his desk), but some characters I think were a little OOC. I mean, when in doubt just remember that the vast majority of them are insane. The Soldier especially. He just seemed too functional.
Also, in regards to what someone said below, when in doubt: Just because something is MEANT to be annoying, doesn't mean it's any less pleasant to watch.