It's a $20k, street-legal, EV modding platform. Sounds like you can mount your own infotainment system. Just an electric motor, battery, and chassis, and the rest is up to you. Isn't this what we've been asking for?
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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.