Energy can be transformed from one form to another and sound is a form of energy in which the molecules in matter--solid, liquid or gas begins to vibrate. This occurs when a force is exerted onto an object that starts the object vibrating and transfers energy through sound waves. These waves are mechanical waves and sound is measured in units called Joules (J).
When someone is speaking, the vocal cords are vibrating, which in turn vibrates air molecules around them to create a sound. The vibrating molecules are a stimulus that in turn vibrates our eardrums, and eventually, fluid in the ear vibrates, then a nerve impulse is sent to the brain and we hear a sound.
Playing a drum is an example of sound due to vibration because a force is applied to the skin on the drum which in turn, vibrates and the air above it vibrates and eventually, the ear will sense these vibrations as sound.
Another is example is the sound of thunder. Lightning is due to a discharge of electrical energy and most of it is converted to heat energy and a small percent is converted to the sound we hear as a thunder clap, the rest is converted to light energy. Because there is a rapid temperature and pressure change in the air during a thunder storm, it expands violently and this rapid movement causes the boom we associate with thunder.
As long as there is an object to vibrate--in the form of solid, liquid or gas, sound waves can be produced. There are no sound waves in a vacuum however.
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