It's part of an ancient mystery I haven't been able to solve yet.
4 seconds sound clip from Scooby-Doo, Where Are You! - Scooby Doo and a Mummy, Too (S1E12).
You can hear this line at 00:02:41.727 in the Blu-ray version of the episode.
Quote context
[...]
- Now, if you'll excuse me, Professor, it's time for me to take my leave.
- Well, thank you again, Dr. Najib.
- Well, do you kids still want to stay and help me tonight?
- It'll take more than a silly superstitious curse to scare us off.
- Hey, Professor, what kind of a crazy coin is this?
- It's part of an ancient mystery I haven't been able to solve yet.
- Well, could you solve how I can get a sandwich? I'm starving!
- Me, too!
- Why don't you kids run downtown and bring back some sandwiches? I'll buy.
[...]