One of the coolest features on the old collector's edition laserdisc of John Carpenter's Halloween (and maybe its turned up on one of the bazillion Anchor Bay DVD versions and I just haven't seen it) was a piece highlighting how important music was to a finished film. In it, they showed a scene of Michael stalking Laurie without the iconic soundtrack and then with it. The difference was amazing and certainly gave me a newfound appreciation for how the score could affect a film.
Another area of cinematic sound design that's often overlooked is voice-acting. Generally speaking (pun fully intended) actors voice their character's dialogue. However, in the case of George Lucas's Star Wars, a different approach was necessary. Darth Vader is arguably the greatest and most iconic screen villain in the history of cinema -- partially because he's intimidating and awe-inspiring with his Force powers and snazzy black outfit and partially because his voice is ominous and terrifying. However, everyone knows that James Earl Jones didn't physically play Darth Vader. Instead, English actor David Prowse donned the costume. Prowse certainly had the physical stature to make a convincing Lord Vader, but as the clip below demonstrates, his voice wasn't quite as well-suited for the role.