The speed of sound depends on the type of medium and the temperature of the medium. If we consider the atmosphere on a standard day at sea level static conditions, the speed of sound is about 761.2 mph, (1,225 km/h).
We can use this knowledge to approximately determine how long would it take for sound to travel around the world. The circumference of the earth is around 40,000 km.
So, if sound travels at a constant speed of 1,225 km/h, it would take 32.6 hours to travel the distance of 40,000 km.
Air is a gas, and a very important property of any gas is the speed of sound through the gas. The speed of "sound" is actually the speed of transmission of a small disturbance through a medium. Sound itself is a sensation created in the human brain in response to sensory inputs from the inner ear.
Disturbances are transmitted through a gas as a result of collisions between the randomly moving molecules in the gas. The transmission of a small disturbance through a gas is an isentropic process. The conditions in the gas are the same before and after the disturbance passes through.
Because the speed of transmission depends on molecular collisions, the speed of sound depends on the state of the gas.
The speed of sound is a constant within a given gas and the value of the constant depends on the type of gas (air, pure oxygen, carbon dioxide, etc.) and the temperature of the gas.
An analysis based on conservation of mass and momentum shows that the speed of sound a is equal to the square root of the ratio of specific heats g times the gas constant R times the temperature T: a = sqrt [g * R * T]
Get more info about the speed of sound on www.grc.nasa.gov.