A lot of attention gets paid to the camera techniques innovated by The Birth of a Nation, but the film was equally revolutionary when it comes to music.
Like most silent films of the era, The Birth of a Nation was scored differently depending on where and when you saw it. The music was played live by musicians, after all. Although the original score for the film, composed by Carl Elinor, is now lost to time, the most notable version of its soundtrack was created by J.C. Breil.
The score contains a mix of original works, classical pieces, and reinterpretations of American standards. It's super-effective. This might be the first instance of "Flight of the Valkyries" being used to score a battle scene, for example, but it's certainly not the last. The American standards are used in a clever way too, as Griffith uses them to relate the South with American ideals.
The film's original works aren't slouches either. The most notable of these is the ridiculously named "The Perfect Song," which is the love theme associated with Ben and Elsie. It's widely considered one of the first love themes in cinema. What's more, it became such a popular piece of music in its own right that it would go on to become the theme song for the radio show Amos 'n' Andy—which, as it happens, was also pretty racist. Isn't that a coinky-dink?