Here's what one that I got says. Notice that I have to snap my fingers at it to get it to play. Also, he doesn't just talk - he's got his own sound effects and music as well. I think this makes him harder to understand.
Some friends and I worked out how to do this inexpensively; as a matter of fact, this is the least expensive way possible to do this. These talking skulls are usually about $20.00 new, and less off-season. Other versions of this project on the web use parts that cost lots more than the skull!
Here's a quick explanation and demo:
We made a custom-designed circuit that needs less than $5.00 of parts – plus we’ve arranged with a company to offer a professionally printed circuit board so you can assemble it like a kit, instead of working out how to build it on prototyping board. And the cost of the circuit board is only $7.00, and less if you want more than one.
How it works: The circuit inside the skull is replaced by a small, custom-made board that will respond to “beeps” on an audio track; when the circuit hears a “beep” it turns on the motor in the skull, opening the mouth.
It’s a great project paired with a microcontroller (Arduino, EFX-Tek Prop-1, or Picaxe) that can trigger the playback of an MP3 or WAV file.
This is a multiple-part project, and you should have some experience in hobby electronics: Reading circuit schematics, soldering, assembling circuit boards, editing sound files on your computer, and programming a microcontroller to play back MP3s when you want.
2014 update: Someone asked if you could use this live with a microphone, so you could be hidden somewhere, and have a skull that is sitting on a table talk to people and react to what they say. Great idea! Yes, you can - there's a simple way (and it's easier than the original project!) Go to "Testing the skull" and see the notes at the bottom of the page.