Pretty good game you made. Although there could be a few improvements regarding balance as others have said.
Graphics were pretty good compared to the generic zombie shooters that amateurs come out with in the flash portal. Props to you on that. The main focus you should concentrate on next time you make a game should be balance and the peeves of the players. I recommend hiring beta testers(I'm sure there's a good amount of people who don't mind finding glitches for you or simply playing your game and giving you feedback before release) the next time you're about to submit something to the flash portal.
Sounds were fine for the guns. I noticed the zombies only made a sound when they died. Perhaps you could keep some sort of moaning background sound. Throw in some variations for when new zombies are added to the mix.
I beat it in about an hour. Had some fun, although was a bit disappointed I only got to use the PKM machine gun for only 1 wave. :/
It was pretty exhilirating, there was a moderate level of difficulty throughout the game.
I unfortunately had to restart after reaching wave 20 due to the fact I simply had bad luck during loot runs. My survivors only found a pistol after raiding all the houses in the first 2 screen areas, and 1 of them died, so I was left with 2 survivors. Whereas when I restarted, I had 9 survivors by the time I reached wave 20 and unlocked half the guns already. :|
My main concerns are:
Your only survivor that you start off with dying. There should be an "invincible" default survivor just so that the player has a chance to start over if all the other survivors die off. This could be a problem because survivors dying seem to be based on a dice roll sort of method.
Not being able to aim at zombies too far below the screen.
I'm talking mainly about zombies that reach near enough to your barricade that when you try to aim lower, your mouse goes off-screen. Perhaps you could simply make the window bigger? Expand the art a little bit towards the bottom so that players can aim at those zombies who just slipped their grasp.
I notice there's no way to see how much ammo is left in your magazine, thus rendering having a secondary weapon somewhat useless on later levels when you don't know when to take out the secondary, since it's supposed to be for something you take out during reloads. A simple counter would suffice. i.e. 5/5 for a full magazine on the default weapon, 1/5 when 1 bullet left, etc.
I felt the crosshair was adequate, but could've been improved on so that the center is easier to see.