gaunty:
Eh? Wot u been smokin'?
Sound is not what causes aeroplanes to fly. The principles of aerodynamic flight don't apply in space anyway. The noise comes mostly from the engines, either in the air of out of it, and the noise itself is caused by turbulent flow, whether it be over the wing, in the exhaust stream, or in the engine itself.
The main reaqson why you can't hear an aircraft at FL420 is because it's 8 miles away.
Pie