The speed of sound can change quite a bit depending on what it is traveling through (air, water, or even metal). The best way to understand this is to know just what sound is.

When you talk or a when a stereo plays music from a speaker, the air molecules nearby gets pushed away into neighboring molecules and the bumping continues in a wave much like when you drop a rock into water and the waves travel away in a circle. When the last air molecule finally hits your ear, your brain tells you there is a sound.

As for the speed, that depends on how quickly the molecules vibrate and hit each other. Air is a gas and there are relatively few molecules per volume so the sound travels around 773 miles per hour. Water is a liquid which is much more dense than air and the molecules hit each other quicker. So if you were under water and someone hit two rocks together the sound would reach you at a speed of 3349 miles per hour. Finally, if you were to place your ear on a railroad track which is made of steel and someone tapped it with a hammer further down the track, the sound would reach you at a speed of 11,185 miles per hour! That is very fast because steel is a solid material and the molecules can vibrate into each other very quickly.

So when fighter jets go faster than the speed of sound, what they mean is the speed of sound in air. This is called the Mach number.

A jet traveling at Mach 2 is:
2 x 773 = 1546 miles per hour.
The fastest plane can reach speeds of 2193 miles per hour.

One last note. If you were in space you would not hear anything. That is because there is nothing in space that can vibrate to transfer the sound. So the speed of sound in space is 0.