From Altitudes of at least several thousand feet AGL, -its Surprising what you can hear from down below. Especially in Stiller Air. People Talking, Dogs Barking, etc. Just ask many a Ballooner!

I know that the Higher you keep on getting up, -the more Difficult this Audability is.

Just look at how Silent, or Nearly Silent, -that typical Contrail producing Airliners are in their Cruise, -from our Vantage Points down below. True also, -there is also much Thinner Air at such Heights with which to Carry such Sound. But I don't think that this can account for all of such Silence. Sound of course recedes with Distance, and thus Altitude, anyway.

But still, -I'd reasonably think that such Ground Sounds could possibly Carry round 10,000 to 15,000 feet, -but as for Certainty on that, -I simply don't know.

And so on his hearing Gunshots, -Such may well *Not* have been aimed upward toward him.

If on the Contrary they were, -which I again so very much Doubt, -I Second the View that it would have to be Some Kind of Gun!