The car has slowed down because of the deceleration or decelerating force acting on it. The decelerating force acting on it is generated by the application of brakes or the frictional force acting in the opposite direction of car's motion.
Escape velocity is the velocity an object needs to escape the gravitational influence of a body if it is in free fall, i.e. no force other than gravity acts on it. Your rocket is not in free fall since it is using its thruster to maintain a constant velocity so the notion of "escape velocity" does not apply to it.
For a sound wave traveling through air, the vibrations of the particles are best described as longitudinal. Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport. They move the same direction as the wave.
