LONG DISTANCE SOUND TRAVEL

In free space with air of uniform density, sound travels in the far field in an easily predictable manner. It travels with spherical spreading losing a sound pressure level of 6 dB for every doubling of distance. If the sound level at 100 feet from a loudspeaker is 100 dB then 200 feet away it is 94 dB, 400 feet away it is 88 dB.

However the conditions of free space and uniform air density do not often apply in the amphitheater setting. Sound grazes for long distances over fields, housetops. Sound is scattered by trees, walls and many other ground level obstructions. In the grazing process sound level is lost, even if the listener can plainly see the source of sound. This happens in the low frequency bands. Details are found here.

Sound appears to travel long distances better downwind than it does upwind. This phenomenon is not the result of the sound just being "blown along" by the wind. It is the result of the wind vertical gradient in which the wind travels faster higher up. Because of this gradient some of the sound that is initially headed into the sky is curved down earthward, ending up at remote locations.

Additionally there are the effects of temperature inversion. Details can be found through a search of wikipedia. Compare sound refraction with the formation of mirages.

Interesting anecdotal information can be found here.

Clearly sound barriers at the perimeter of the venue do not have any influence on this sky wave.