Pressure waves in the air cause sound waves to form. However, sound waves can also form in water and in solids. The speed of sound varies significantly depending on the temperature and medium it travels through.
Sound is considered a wave, but it is not a form of electromagnetic radiation like light. Instead, sound is a pressure wave that forms when the media it is traveling in is disrupted in some way. Because it needs a medium to form and propagate, sound cannot travel in a vacuum; there is no sound in space.
The speed of sound varies based on the density of the medium it travels through. Sound travels faster in water than in air, and it travels even faster in solids. When a sound wave travels from one medium to another, much of the wave is reflected. Because of this, thin barriers can effectively block most sound.
The speed of sound is lower in the upper atmosphere, which created problems for some early aircraft. Traveling at supersonic speeds places a tremendous amount of stress on an aircraft, and the control surfaces, especially those on the tail of the plane, will cease working if the aircraft is not designed for supersonic flight.