This sound comes from the Metal Slug series, SNK's side-scrolling arcade run-and-gun franchise that began in 1996. The Mission Complete cue became familiar through stage-clear screens and post-level transitions across the games. Online, it spread through gameplay uploads, emulator videos, and retro compilations, then found a second life in short edits as a quick audio shorthand for success. People still use it because the jingle is short, catchy, and instantly communicates that a job is finished.