Perfectly quiet.
What would outer space sound like? Would it be calm and quiet like an astronaut floating in darkness or loud, thunderous lightning storms with crackling volcanoes and fierce whirls of hurricane winds?
Sound is radiant energy carried by high frequency pressure waves through the air, or matter like water or wood.
Since there is no air in the vacuum of outer space, sound waves cannot travel, it is dead silent.
Sound is the movement of energy through substances in longitudinal (compression/rarefaction) waves. Sound is produced when a force causes an object or substance to vibrate — the energy is transferred through the substance in a wave.
Sound energy is a form of energy related with the vibration of matter. The SI unit of sound energy is the joule. Sound is a mechanical wave and as such consists physically in oscillatory elastic compression and in oscillatory displacement of a fluid.
Here's the classic lab demonstration that sound needs a medium in order to propagate:
probably quiet.