You'd better check yours as well
<On Earth, the speed of sound is 340 kph
Ah, no. 340 kph is roughly 210 MPH. I know a lot of private pilots who fly faster than that. A Lancair IVP homebuilt can cruise 335 MPH without breaking a sweat. I'm reasonably certain you meant 340 meters per second, not kph.
The speed of sound on Earth is approximately 760 MPH (~1225 KPH). At 30,000 feet, it's roughly 670 MPH (~1080 KPH). Those figure are from memory.
Here's a calculator for the speed of sound based on temperature and humidity.