The trouble with both is you would need the cab of the car to be sealed to the point you could pull a vacuum on it. Any air leaks would throw off the readings. then again what you are reading is air pressure changes within that sealed cabin. The lower octaves will have the biggest effect on the readings. You'd be able to tell when bass notes are played but you wouldn't be able to differentiate what frequency it was. You wouldn't be able to measure treble (or at least I don't think so).
Then again, everything I just typed was a thought experiment and I could be wrong.
