The speed of sound is roughly 767 miles per hour depending on temperature and humidity. How is this a subsonic flight?
The aircraft was flying at its regular subsonic rate, while the air it was in was travelling at 250 mph in the same direction. The airspeed was subsonic, while the ground speed was 825 mph.
If you were with Nolan Ryan on a plane going 670 mph and Nolan went to the back of the plane and fired off a 97mph fastball towards the cockpit, would the ball then be traveling at the speed of sound?