Sound is a form of acoustic energy which propagates through matter. All sounds are vibrations of molecules which move as sound waves. It can be heard by a being or device that can interpret it. Its intensity is measured in decibels. The pitch of sound is measured in hertz.
Sound in Space Edit
Sound requires solids, liquids or gasses to travel through. Space is a vacuum so there's no matter through which sound can travel. The speaker array inside the cockpit of a ship simulates all audio happenings in real-time. When the canopy breaks, all audio no longer has air to resonate with. Then it becomes quiet and you only hear yourself breath in your Remlok Suit.