Best rough figure I could find for weight of a loaded 737 is around 40 tons, frontal area I'm guesstimating is roughly a 3m diameter cylinder plus ~0.5m average thickness of the 28m wingspan giving 21m^2? For which the calculator gives 262.8m/s or 943.2km/h.
"The Boeing Co. 737-800 was knifing through the air at more than 640 miles (966 kilometers) per hour, and at times may have exceeded 700 mph, according to data from Flightradar24, a website that tracks planes.
"The preliminary data indicate it was near the speed of sound," said John Hansman, a Massachusetts Institute of Technology astronautics and aeronautics professor who reviewed Bloomberg's calculation of the jet's speed. "It was coming down steep.""