One person who noticed the same distinctive scream reoccurring in so many movies was sound effects fan Ben Burtt. Ben and his friends in the cinema department at USC, Rick Mitchell and Richard Anderson, noticed that a scream was popping up in a lot of movies. One of the films they made together, a swashbuckler parody "The Scarlet Blade" (1974) included the scream - which they borrowed off another film's audio track.Link, Link to video (Thanks, Matthew!)
A few years later, when Ben Burtt was hired to create sound effects for Star Wars (1977), he had an opportunity to do research at the sound departments of several movie studios. While at Warner Bros. looking for sound elements to use in the space adventure, he found the original "Distant Drums" scream - which he called "Wilhelm" after the character that let out the scream in "Charge at Feather River."