One of the ideas we brainstormed at the Mapper-Modeler conference this past spring in Zurich was the idea (often suggested before, apparently) of having sound in Google Earth.
Check out the YouTube clip above demonstrating what that would look like (and sound like), or try it out here in an embedded Google Earth window.
I think it adds a lot of realism to the buildings and terrain without adding a lot of heavy polygons. What do you think?