Under normal atmospheric and altitude conditions, a jet breaks the sound barrier at approximately which speed?

I just had the question when the first airliner went mach 1 (660 mph) in 1961. Speed of sound getting faster? No. It's a mixup of units. Mach 1 = 760 mph = 660 knots/hr. Why anyone would count the knots out of anything but a seagoing vessel is beyond me.