I seem to recall that there are:

a) Toys you can purchase in the store that already do something similar. Such as a moose head that talks to you when you walk by, or a fish that sings when you press a button.

b) Sites on the internet which describe how to "mod" these toys to flash your own sound files into their chips. I would expect that some of these toys would be easier to hack than others.

You said you searched for items that would do this sort of thing pre-made and didn't find any. But I'll bet if you searched for sites that talked about hacking a preprogrammed toy to do your bidding, you'd find something.

Anyone got links?