Sound is vibration, or energy, that gets all the molecules around the source of that vibration all excited, so they vibrate too and start bumping into each other and pass along the vibrations. It's kind of a molecular mosh pit. The molecules lose some of their excitement the further away they are from the source of the vibration so the noise gets quieter and quieter until the molecules run out of vibrations and there the sound ends. Sound waves are essentially variations of pressure that exists, in this example, in the air. (Air is made up of molecules).