Sound waves are pressure waves in physical media, be it solid, liquid, gas, or plasma. Sound waves and pressure waves in general, have been important since the formation of the universe, since the movement of anything in any kind of medium (or in contact with a medium) will generate sound waves.

Many groups of animals can detect sound, although hearing among different groups of animals might be evidence of parallel evolution. In any case, sound was already have used as a sense 540 million years ago. Sound has also been used for other purposes than merely detection; insects we know use it for signaling, such as for mates (440 million years ago), and other groups may have used it even earlier. There is some suggestion that the very early mammals (200 million years ago) first used sound for echolocation, but the first case we know of this, the first fossil bat, is about 55 million years old. Orcas (killer whales) actually use sound waves as a weapon for killing prey, although this is soft-tissue so I cannot say how long ago that evolved.

Sound is the vibration of any substance. The substance can be air, water, wood, or any other material, and in fact the only place in which sound cannot travel is a vacuum. When these substances vibrate, or rapidly move back and forth, they produce sound. A stereo speaker, a violin, or our vocal chords for example, can produce sound when they vibrate. The vibrations occur in the molecules of the substance, and the vibrations move through the substance in sound waves. As sound waves travel through the material, each molecule hits another and returns to its original position. The molecular movement causes the energy of sound.

We receive air vibrations into our ear, and it sends electrical signals to our brain where it is translated. The vibrations are transformed into audible sound (from 20Hz to 20,000Hz, being Hz the unit of the sound waves frequency) by our brain. The transformation is from mechanical energy (movement) into electrical energy. The sound energy has a big impact in several kind of equipment for medical diagnostics and treatment (ultrasound, with frequencies above 20,000 Hz), and navigation systems (radar). The sound energy also has an impact in our life as it is one of our main ways of communicating with the rest of the world. This energy became more useful when the way of transforming it was discovered (transducers), in order to get different applications of it.

Sound is an energy form that consists of pressure waves propagated through solid, liquid, gas or plasma media. Mostly you don't see overt effects of sound at 'normal' intensities because the sensitivity of you hearing is so acute-- noisy sounds impinging on the ear have power levels in millionths of Watts-- and quiet sounds are billions of times lower power. However, at higher energies, sound can transform in many ways -- sonoluminescence is the direct production of flashes of light caused by conversion of sound energy into tiny bubbles which heat a tiny bit of gas to plasma temperatures-- creating flashes of light. A speaker converts electrical power into sound and a microphone converts it back into electricity. (A piezo-starter on a lighter or gas grill takes the sound energy of a mechanical spring and creates 10-20kV sparks).

There have been several proposals to use sound as a means for pumping refrigerants for heat-pumps and air conditioners, but so far they have not proven practical. (On issue is that there are no perfect sound conductors or insulators in reasonable materials-- making devices hard to isolate and make efficient.) One form of 'sound' is quite prosaic in the world -- earthquakes consist mostly of pressure and transverse waves at low frequencies -- and they can pack truly enormous power, despite being too low a frequency for us to hear.