Well, you probably can't download them anywhere on the internet (at least not that I know of, I've never looked into it), but the game has to store the sound files somewhere on the hard disk obviously. Unfortunately most games these days pack their important game files into a package like they do in Half-Life, so the only way of accessing these files is probably with some user-made program, in which case you're at the mercy of the game's mod community. If you're really lucky, you'll extract the file and it'll be in some recognizable format that you'll be able to convert to a WAV. If not, I dunno what to do, really.
According to Wikipedia, Postal 2 uses Unreal Engine 2, so you're probably in luck as far as file formats goes. That's the most help I can give you though, as I know little of anything about the Unreal engines. You could probably find that with a couple Google searches though.