The Good, The Bad, and The Ugly has one of the most iconic movie soundtracks of all time, with an extremely memorable theme song. The theme song has become such a huge part of pop culture that it has become almost synonymous with cowboys.
The soundtrack was scored by Ennio Morricone, who was an Italian composer and conductor. He worked with the films director, Sergio Leone to ensure that the music would help the narrative flow smoothly. His music was described as “tunes that are as vividly in the foreground as any of the actors’ faces,” by The New York Times critic Jon Pareles.
The iconic whistling at the beginning of the theme was created to mimic the sound of coyotes howling. Morricone described that the “wah-wah-wah” sound came from the combination of two hoarse male voices and added reverb. He then said the sounds were inspired by brass instruments like trumpets and trombones when the move “the damper back and forth.”
Source: https://www.wideopencountry.com/the-good-the-bad-and-the-ugly-theme/