Why do you hear a thunderclap when the sound barrier is broken?
If you throw a ball at 10 Km/hr when stationary, the ball will begin travelling at 10 Km/hr, as it travels through air friction will slow it and it will come to a halt eventually. Let us say you were travelling in a car at 60 Km/hr and threw the same ball, depending on which direction you threw the ball in, it will either travel at 70 Km/hr or 50 Km/hr (for the sake of simplicity, I am not considering vectors).
Unlike the ball, sounds tends to travel at a constant speed depending on the medium that it is travelling in. Also, it is a pressure wave unlike light, which is a transverse wave. The wave leaves the the source and moves at a constant speed through the medium, which may be air, water or even a solid.
This is what a light wave looks like.
This is what a sound wave looks like.
Generally sound travels faster in solids compared to liquids, faster in liquids than in gases. Sound travels at about 1235 Km/hr in air at sea level. It travels a little slower as you go higher since air becomes rarer.
Unlike the case of the ball mentioned above, sound being produced by a jet engine will travel at a constant speed. When the jet begins to approach the speed of sound, every successive pressure wave begins to arrive faster and faster. When a jet reached the speed of sound, the next sound wave arrives before the previous one has been able to move.
Its like someone coming running from behind and slapping you.
This results is what is called interference. In this case constructive interference. The two sound wave collide with one another resulting in the Sonic Boom.