I think you would want to map this for like hl2 or something. Css is not meant for this.

You can't CSS is a multiplayer only game, and does not support NPCs.

Maybe bots, or trigger_hurt tied to a zombie model, when zombie model is tied to something that moves by it's self.

It would be extremely tedious, time consuming and end result wouldn't be as good in cs:s as hl2.

There is no real AI implementation in CS:S except for bots.

You would have to have spawn points in each room for the bots, triggers to make the bots spawn, fake an objective for them to find/follow, disable the spawn points from room to room, and lastly: make the map/game not suck.

The easiest part of the question is how to make it play a custom sound. Look in the tutorials section.

I would suggest adding target like brushes on to tracks to make a shooting gallery or something.

I've already tried working on this for a zombiemod tutorial. But the targets were aombies atached to ropes. When you shoot them, they fly away. And the ropes were atached to tracktrains. You won't get much in CS:S. Unless you remember your Gmod days and make it like you would in Wire.