While I was doing some other research, I came across this video from Smithsonian Magazine. It's a recording of Confederate veterans in the 1930s giving the old "Rebel Yell," as well as they still could at what was then an advanced age.
Historians have been arguing for some years about both the actual sound of the yell, and its origins. The most popular arguments are that the South had learned to use it from fighting the Indians, which is plausible because those wars immediately preceded the generation that fought the Civil War; or that it was native to the Southerners because it was derived from the Scottish Highlander war-cry. The latter argument is plausible because the Highlander yell is well-attested, and because of the prominence of Scots among Southern families -- although that prominence is greatest among the Appalachian Southerners, who were least likely to support the Confederacy.
Interesting to discover that there's an actual recording, then!