In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: sound travels most slowly in gases; it travels faster in liquids; and faster still in solids.
Sound travels at 330 m/s in air
it travels at 1,550 m/s in water (4.3 times as fast as in air)
at 5,120 m/s in iron (about 15 times as fast as in air)
In an exceptionally stiff material such as diamond, sound travels at 12,000 metres per second (39,000 ft/s), about 35 times as fast as in air, which is around the maximum speed that sound will travel under normal conditions.
In the vacuum of space, there is no air, so there is no sound!