No sound in a vacuum. Remember the scene in the film 2001 where the astronaut fires the emergency bolts on the space-pod, and is blasted into the the spaceship airlock through the vacuum of space - without his helmet? As he is flung into the chamber, he hits the emergency pressurisation lever. At first, it being a vacuum, we hear no sound, but gradually, the hiss of the gas is heard as the chamber fills with air, until it becomes a roar as the air pressure builds to normal. Technical excellence!