If you can live with a wire, how's this:
Wire
One cheapo mp3 player that you can find in magazine stores or cvs, a 20 or so foot stereo cable wire (radioshack) and a speaker in or behind the puppet. you connect the wire from the earphone output on the mp3
player to the input in the speakers. You make the laugh the only track on the player and when you hit the button, he laughs.
Wireless
As far as wireless, I am almost positive radioshack had wireless speakers of some sort, but for sound quality and consistency, I'd argue you'd. need a wire. Otherwise it's the same idea;mp3 player wirelessly rigged to the speakers
Motion sensative
I don't have a clue how you'd make one that was motion sensative. Maybe taking apart one of those prebought ones (again cvs) might help, but you'd need to ask someone else how to connect a new sound chip to the board.