Your vocal chords and the strings on a guitar are both sources which vibrate to produce sounds. Sound energy is in the form of waves, which travel outward in all directions from the source. Without energy, there would be no sound. Let's take a closer look at sound waves. Waves are made up of compressions and rarefactions.
Compression happens when molecules are forced, or pressed, together.
Rarefaction is just the opposite, when molecules are given extra space and allowed to expand. Remember that sound is a type of kinetic energy. As the molecules are pressed together, they pass the kinetic energy to each other. Thus sound energy travels outward from the source. These molecules are part of a medium, or anything that carries sound.