I do not know if these sounds are real or not. I do know that the U.S. govt. was working on a protype plane called the Auroa project that used a methane pulse propulsion design. It was designed to fly at 10,000 to 12,000 miles an hour. I would imagine that if the Aurora project had taken flight this would be what it would of sounded like. It was said that it would fly at an altitude of 150,000 to 200,000 feet and creates a ground measured earthquake of 2.0 to 3.5 as it roars thru the sky. Witnesses say it sounds like the sky is being ripped open. It could fly to any point in the world in under an hour!
Again this is hypothetical, this plane was never "officially" built.