I think that everyone is forgetting the sound of the final shot doesn't eco and just flatout stops in the middle.
There are a couple of requirements for an echo to occur naturally. Echoes happen because sound bounces off something, which I'm sure you knew. The other requirements are
distance from the object that causes the sound to rebound, and the
volume of the original sound. According to my sources, a really
good echo won't occur
in nature without some smooth reflective surface
no closer than approximately 225 feet/75 yards from the originating source. Then the sound must also be of sufficient
strength/volume to reach the reflective surface (call it a natural "wall" if you like) with enough residual strength to enable a large percentage of the sound to be reflected. I have no idea what was in the woods that may have caused an echo, but there was probably nothing 225+feet above the shooter's head to reflect any sound, hence no echo effect. There was
undoubtedly enough
strength/volume in the discharge to have created
one condition for an echo. Pax...
(Technically, and contrary to a popular belief by some, a duck's "quack"
does echo - but it may not be
separately distinguishable from the original quacking sound.)