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from Wikipedia:

The Wilhelm Scream is a near famous stock sound effect first used in 1951 for the film Distant Drums. Actor-singer Sheb Wooley is considered to be the most likely voice actor for the scream, having appeared on a memo as a voice extra for the film.

The Wilhelm Scream has been featured in many films, television programs and video games since. Alongside a certain recording of the cry of the Red-Tailed Hawk, the “Universal Telephone Ring,” the Goofy Holler, the Tarzan Yell, “Castle Thunder,” and the Howie Scream, it is probably one of the best-known cinematic sound clichés.

The Wilhelm’s revival came from Star Wars series sound designer Ben Burtt, who tracked down the original recording (which he found as a studio reel labeled “Man being eaten by alligator”). Although Distant Drums was the first known use of the sound, Burtt named it after “Pvt. Wilhelm,” a minor character who emitted the same scream in the 1953 film The Charge at Feather River.

Although the identity of the individual who recorded the scream (which was actually one of a series of six) remains unknown, researcher and Star Wars sound effects editor Ben Burtt uncovered documentation suggesting the scream might have been recorded by singer Sheb Wooley, who was one of the actors in Distant Drums.