The cha ching sound comes from old mechanical cash registers, whose bell and drawer made a two part clang when a sale was rung up. That noise became slang for money being made, and the phrase spread through cartoons, adverts and everyday speech long before memes. Online, creators stretched the sound for comic timing, turning it into a shorthand for profit, selling and splashing cash. It stays popular because almost everyone recognises the register cue instantly, even without seeing any money on screen.