I don't know. Boy, this mystery's getting more mysterious by the minute!
4 seconds sound clip from Scooby-Doo, Where Are You! - Go Away Ghost Ship (S1E14).
You can hear this line at 00:12:55.425 in the Blu-ray version of the episode.
Quote context
[...]
- Shaggy!
- Scooby, your voice has changed.
- Up here!
- Velma!
- Don't worry, we'll save you! I think.
- Come on, Scoob... follow that ghost ship!
- So this is where they've been hiding.
- A secret cove at Skull Island.
- Why does a ghost ship need a secret cove to hide in?
- I don't know. Boy, this mystery's getting more mysterious by the minute!
- There they go! Keep paddling, Scooby!
- Keep paddling and quit clowning around!
- Ha ha ha! Soon you shall join my crew! Ha ha ha ha ha!
[...]