In the future, I suspect we'll see aftermarket outfitters touting externally facing audio equipment to imitate the sounds produced by high-powered combustion engines.
I also suspect the majority of them will be sold to teenagers with fairly under-powered vehicles ;)
The BMW M5 (an example of exactly what the Model S is competing against) already does this, because the interior cabin is so well insulated that you can't hear the turbocharged V-8, and owners want to: http://reviews.cnet.com/8301-13746_7-20111031-48/bmw-m5-gene...
Actually this exists, but not for cosmetic reasons. Hybrid cars on their motors are just so silent that they're worried that someone (probably someone blind) will be hit by them.
This already exists. The Nissan Leaf emits some cool sci-fi sounds by default, and I heard a dealer mention it can be replaced with a large selection of sounds by specialized auto shops.
Closest you'll get is turning on the AC. Because the car's so quiet, I can hear (and feel) the rumble of the compressor running when I turn down the temp in the car.
