When do the sounds correspond to those that are heard behind the viewer (you)? Just curious if it is for something coming in out of the field of vision behind you, as you can get a very wide stereo image in 2.0...
The sounds heard to the side and behind are best represented by surround speakers. A good 5.1 system with the surrounds located slightly behind the listener will present a wide rear stereo image just like you get from the front. In my room, my side surrounds are slightly in front of the listening position so a 7.1 setup works best for me.
The sounds in the surround channels represent what what is physically beside or behind the listener as you watch the movie. In Bug's Life, you get the insect noises (caterpillars eating cilantro
), leaves rustling, wind noise, rain and much more that make you feel like you are in the middle of a field. The goal is to make you feel like you are in the action rather than with your back up against a cliff and everything is happening in front of you. Here are some of the many things you will hear behind you: gunshots, footsteps, engine noises, city noises, planes & helicopters flying overhead, an entire army behind you as you advance forward in a war movie, explosions, hoof beats, etc. It is just like being there in real life.
Do they add ambiance so it feels like you are in the room with the off camera "extras" mumbling behind you?
No, it makes you feel like you are "in" the movie rather than watching the movie. There are two ways to listen to books on tape/cd. One is with the the book just read by one person and no extra sounds. The other is with the book read with different readers for each character and the full assortment of sounds. For example, I have the BBC dramatization of the Lord of the Rings trilogy (from 1981). This made me feel like I was in the action as opposed to reading a book. Surround does the same for movies: it puts you into the scene.
Again just curious as I'm only 2 channel, I assume some movies benefit more than others?
Yes, some movies benefit more. I think action/adventure movies probably benefit the most.