Wikipedia [1] answered my question of why this is called the Wilhelm scream:
> The sound is named after Private Wilhelm, a character in The Charge at Feather River, a 1953 Western in which the character gets shot in the thigh with an arrow.
> The sound is named after Private Wilhelm, a character in The Charge at Feather River, a 1953 Western in which the character gets shot in the thigh with an arrow.