Well, it's kind of a slippery number, because the speed of sound
depends on the air that the sound is traveling through, so it's actually
different for different aircraft flying at different altitudes, where the
pressure, temperature, and density of the air are different.
Here are some "standard" values"
On the ground . . . 761 mph
10,000 feet 733 mph
20,000 feet 705 mph
30,000 feet 677 mph
50,000 feet 660 mph
100,000 feet 676 mph