Several factors can affect the speed of sound, like air temperature, the material the sound wave is passing through, and the sound wave frequency, for example. On Earth, at sea level, given an air temperature of 59 degrees Fahrenheit (15 degrees Celsius), the speed of sound is 761.2 mph (1,225 km/h). Sound moves faster through warmer air. As such, the higher in the atmosphere you are, the lower the required speed is to break the sound barrier.