Mach 0.7 at 40,000ft is 407 knots. The speed of sound, which of course changes with altitude (well technically temperature), travels at 573 knots at 40,000ft. For a 737 to reach the speed of sound at 40,000ft, it would need to be cruising at Mach 1.0, which is completely unrealistic for a normal passenger 737.

No, a Boeing 737 is not designed for supersonic flight and cannot reach Mach 1 (the speed of sound) under normal operating conditions. Its airframe and engines are optimized for subsonic cruising speeds, typically around Mach 0.78 to Mach 0.82. If a 737 were to enter a steep, uncontrolled dive from a high altitude, it might briefly reach "transonic" speeds (near Mach 1), but the aerodynamic forces would likely cause catastrophic structural failure. Unlike supersonic jets, the 737 lacks the thin, swept-back wings and specialized engine inlets required to manage the shock waves created by breaking the sound barrier. In 1991, a China Airlines Boeing 747 (a larger cousin) famously survived a terrifying 30,000-foot plunge where it reportedly approached supersonic speeds, but the aircraft suffered massive structural damage that rendered it nearly scrap. In short, while gravity could push it close, the plane would likely tear itself apart before successfully maintaining supersonic flight.