We're betting the musical theme to this movie is unlike anything you've ever heard before. It contains musical instruments and styles that represent all kinds of different things in the movie.
For starters, it begins with a drum that sounds Native American in style, which might make you think of Chief Bromden (who turns out to be the hero of this movie once McMurphy is dead). Then you hear this strange singing sound come in, which is actually the sound of someone playing a handsaw with a bow. The sound of this saw is haunting, but also kind of maddening, just like the experience of being inside a mental hospital.
On top of the two main instruments, there's a set of background instruments that swell as the song goes on, making it sound more and more triumphant as the volume goes up. This prepares us for the moment when Chief finally claims his freedom and jumps out the window of the mental hospital. Give the song another listen and you'll realize just how perfect it is for this movie.