It depends on whether the car has a closed cabin (as most cars have) or whether the cabin is open (as in many sport cars). In the first case, the air in a cabin moves with the speed of the car. This means that it is stationary in respect of the driver and the passenger, and there will be no change in comparison with the stationary case, i.e. the driver will hear the rear passenger perfectly. The case of open cabin is much more complicated. Even in the idealised case of fully open cabin, when one can assume that the air remains stationary in respect of the ground and both the driver and the passenger generate supersonic Mach cones, it is a too small distance between the driver and the rear passenger in order to give a definite answer. If the distance between them were large enough, say 20-30 m (which, of course, is impossible for real cars), then it could be said that the driver would not hear what the rear passenger says. In a more realistic situation of a short distance between the driver and a rear passenger, the rear passenger is in the wake of the Mach cone generated by the driver, so that a more comprehensive analysis, perhaps a numerical one, would be required in this case.