The rebel yell was the war cry of attacking Confederate troops. In books about the civil war it will almost always be discussed, with the caveat that nobody knows what it sounded like. It has been described as being like an animal's cry, derived from the Scottish Highlander's war cry, and often as a sort of Indian whoop.
What's interesting, in spite of all the mystery lavished on it, is that it has been recorded. The end of Ken Burn's The Civil War has perhaps the best example. In it, during footage of a reunion at Gettysburg, Confederate and Yankee veterans are shaking hands over stone wall. A whooping sound starts, and on of the Confederate veteran's turns to the camera and says, "that's the rebel yell."
The clip above is that scene. The person who put it together ends the clip by overlaying the whoops to create a chorus of what it would have sounded like being yelled by a mass of men. Of course younger lungs would have given it more punch, but you can still imagine how intimidating it would sound if you were on the receiving end of the charge.
Along with the video above, there is audio recording of a rebel yell. This recording is of a single veteran demonstrating the rebel yell and can be listened to at the 26th North Carolina Regiment's website. It has a different sound, more of an animal yipping sound than the Indian-type whoops of the video.
There were regimental differences reported in the rebel yells, and this would explain the differences. However, the pulsating rhythm of it is the same between the two, and perhaps that is the key to it,. At any rate, that's as close as we're ever likely to get to hearing a rebel yell, but at least we have some idea how it sounded after all.