"gary pearson" wrote

I don't know if they're audible to the human ear at that height, but you
probably couldn't hear them over normal background noise anyway.

Not a chance at FL 400 and very unlikely even at FL250.

Hogwash. I hear them every day. As a matter of fact, I hear them about
20 miles after they pass overhead. You have to look where they might be
now, instead of straight up where you hear the sound.