Traveling in a car at the speed of sound, you blow your horn at the car in front of you also traveling at the speed of sound. Will the car in front of you hear the horn? Will he give you the finger if he does?
The sound will reach the car in front of you. Although it may seem as though it would not, considering that you are already traveling at the speed of sound, therefore you may think the sound waves go as the same speed as your car therefore not traveling any faster than you are and not getting to the car in front of you. However, this is not the case. This can be explained using Einstein's theory of relativity.
Einstein describes "frames of reference" and in your frame of reference, you are not moving; everything else is moving. Everything in your car is stationary relative to everything else, this is why you don't feel like you're moving, and if you throw a book sideways, it doesn't fly backwards with the speed your vehicle is moving, this is because the car, yourself, and the book are all moving with the same speed your car is moving. They are all stationary relative to each other. When the sound waves are created, they move in all directions at the speed of sound (331 m/s), so in your frame of reference, they would be moving at 331 m/s away from the car in all directions.
This means from the frame of reference of a person standing outside of your car, the sound waves would actually appear to be moving at twice the speed of sound. This is just like if you threw a ball forward out from the car at 15 m/s, it would appear to an outsider to be moving at that speed plus the speed of sound(well in a vacuum anyway, or before air resistance took over) To use a better example of these frames of reference, you know that the earth is rotating. It is rotating at 1070 mi/hr at the equator, yet it seems not to be moving at all, it is our frame of reference when we are standing 'still.' the speed of sound is no different traveling in different directions on the earth because of this rotation.
and yes, he would probably give you the finger. I mean, he is going at the speed of sound, how impatient are you?