Check out this interview with the person who designed the ambient sound system for Disney World.
More than 15,000 speakers have been positioned using complex algorithms to ensure that the sound plays within a range of just a couple decibels throughout the entire park. It is quite a technical feat acoustically, electrically, and mathematically.
How Mr Q Manufactured Emotion [article removed or relocated, check Wayback Machine]
(hat tip BoingBoing)