A lot of Bluetooth speakers today can fill a car with a sound wall better rear speakers used to. Apple says you just need two of their Bluetooth speakers to fill a room in a house with great stereo and reasonably good surround sound. The square footage of a car is generally smaller than the supported room size.
edit to add: if Slate is successful, I wouldn't be surprised if a decently sized ecosystem pops up around easily installed custom sound systems and the tablets (possibly with headphone jacks!) to control them.