It's a violin case. It feels kind of heavy.
4 seconds sound clip from Scooby-Doo, Where Are You! - The Backstage Rag (S1E9).
You can hear this line at 00:02:40.731 in the Blu-ray version of the episode.
Quote context
[...]
- Mmm mmm! I always wanted to be a pizza chef. Watch this.
- Well, thanks for leaving my finger.
- Hey, maybe that's the gang in the Mystery Machine.
- Hey, mister! You lost something!
- I guess he didn't hear me. Let's go see what it is.
- It's a violin case. It feels kind of heavy.
- Yeah, I know. And you never had a lesson in your life.
- Holy moly! What's this? It's full of money! There must be thousands of dollars here.
- Me?
- Guard that while I go phone the gang.
- Uh-oh. Trouble. What's wrong?
[...]