Scooby-Doo, Where Are You! - Season 1 Episode 9 -

As Scooby-Doo and Shaggy are walking home from a nearby pizzeria, they find an abandoned violin case along the sidewalk. Thinking something foul is afoot, Shaggy goes and calls the others so they can investigate. They soon make their way to a local theater and discover it's the base of a group of counterfeiters.