At 20°C (68°F), the speed of sound in air is about 343 meters per second (1,235 km/h or 767 mph) . Therefore, the speed of sound in kilometers per hour is 1,235 km/h. To convert the speed of sound to kilometers per second, we can use the conversion ratio of 0.343 kilometers per second/speed of sound. Therefore, the speed of sound in kilometers per second is 0.343 km/s.