Wow. I first saw this on Chris Pramas' Facebook. The Museum of the Confederacy (a really cool place to visit for history buffs, by the way) has recordings from Confederate vets in the 1920s and 30s trying to emulate the "Rebel Yell". From these, they've recreated the sounds of the Yell as it may have seemed from a mass of angry Rebel soliders. It doesn't sound like what I thought it would sound like, but when you get hundreds or even thousands of people screaming at the same time, you see why that would scare the crap out of the Yankees.
If I ever get to run my Civil War Spies campaign, I need to get that on the soundtrack.
(Sorry to get your hopes up, Billy Idol fans).