Acoustic Shadows are not unique to the Civil War, but that time period is the first instance I have ever seen them in primary sources or historical works.
They are a strange phenomenon related to how sound travels along the contours of geography. For whatever reason, all of the variables which affect how sound carries--density, wind, the presence of hills, trees, rivers and valleys, etc.--Would sometimes redirect or muffle the sounds of a fierce and very loud battle that was very nearby. Some soldiers at Gettysburg mentioned this in their diaries, without naming it acoustic shadows, that they were marching only a few miles from the battlefield yet it was completely peaceful until they cleared one ridge and then were able to hear it very clearly. Meanwhile, other troops were reporting hearing the sounds of battle from miles and miles away.
It is as though the sounds pass right over some valleys and ravines to reach others far away, but without having reached some places very nearby. It was one of the hundreds of oddities and strange occurrences that take place in every war.