Oh and just to clarify for everyone, almost all Electric and Plug-in hybrid cars have external speakers to make the presence of an electric car known to pedestrians, they have for years. It's just that this is the first BMW to have it since BMW, Tesla, and VW have chosen only to add noise where required by law up to this point. It's usually only on when the car is in electric-only mode.
http://en.wikipedia.org/wiki/Electri...warning_sounds