Lasiorhinus - there is no "sound barrier" as such....., an aircraft travelling at any speed above Mach1 will produce shock waves which can be heard on the ground as a boom. Fly high enough, and by the time they reach the ground you can't hear anything. The restriction of commercial supersonic flying over land is more of a political one.
NOX isn't as bad as CO2?! Wonder how they'd safely store liquid hydrogen?