You can't script anything in C. So 3 and 5 are no. You can add sound sources. And wouldn't an invisible and passable wall just be nothing? Well, you can make both invisible and passable walls, but both at the same time would be useless. You can have models, but you'd have to make the collision for them with brushes. Oh, and they should be in .md2, I believe.
EDIT: And hopefully jits, no turrets ever.
Also, like jits said, you can make things move with func_trains, but you can't script them.