The speed at which sound propagates (or travels from its source) is directly influenced by both the medium through which it travels and the factors affecting the medium, such as altitude, humidity and temperature for gases like air. Bad news for Star Wars fansthere is no sound in the vacuum of space because there are too few molecules to propagate a wave.
It is important to note that sound speed in air is determined by the air itself. It is not dependent upon the sounds amplitude, frequency or wavelength. To calculate the speed of sound in dry air at sea level, use the following formula:
V = 331.4 + 0.6Tc
V = velocity (m/s), Tc = temperature in Celsius.
Use the interactive Speed of Sound calculator below to see the effect of temperature on sound's velocity.
For comparison, the speed of light in a vacuum is 299,792,458 meters per second or 186,000 miles per second (669,600,000 mph), which is roughly 870,000 times faster than the speed of sound. The difference between the speed of light and the speed of sound is why you see lightening before you hear it (unless you are struck by it, in which case it may be simultaneous!).
As a comparison with sound in other mediums, the speed of sound in helium at 0°C is approximately 972 meters per second (m/s), or around 3 times as fast as in air. Sounds travels even faster in liquids and solids because of their greater density (although standard measurement is slightly more complicated because of it is effected by the shape of the material and also the fact that both longitudinal and transverse waves may propagate in solids). Sound in 20° C water travels an average of 1482 m/s, while sound in aluminum travels at 6420 m/s. Modern navies depends heavily on being able to predict the speed of sound in varying water conditions for their SONAR echo-location systems.
If you have ever lived in an apartment with shared walls and a party next door, you are no doubt aware that sound waves can transfer from one medium to another and back againin this case from air to the rigid wallboard and back to air again. The speed with which it travels changes accordingly with the shift in mediums. A wide variety of sound isolation products and construction techniques attempt to mitigate this sort of transfer. For further sound-abatement information, see Auralex's Acoustics 101.
It is important to realize that while all sound in the equivalent conditions travels at the same speed, whether louder or softer, higher or lower, the speed or force of the individual molecules bumping into one another increases with amplitude and/or frequency.
For further study, see Hyperphysics->Speed of Sound