I get a different article now too. Even with scramjets, the combustion process is extremely loud. To get the scramjets to an operational range, they would still need something on the range of M4+. That's faster than the SR-71. They must be counting on some sort of atmospheric effect at very high altitudes. I know there is what is refered to as an acoustic duct at about the 20 km (65,000 ft) altitude mark.