Well, granted it has been a few years. But I recall at one time Radio Shack selling those recorder keychains where you could record up to 30 seconds of dialog and play it back. I believe these are still around (sometimes you see them advertised on TV, but they should be readily available) and more widely available now for cheap prices. Plus, they are pretty dang small. A friend of mine once built a Futurama Bender and mounted one of these in the base (he had a button and a speaker grill in the base to amplify the sound). So when somebody would press the button, Bender would say "Bite my colossal metal ***!" That is all it would say, but it got the point across.
I believe you can get programmable sound chips now as well, but it would take a little wiring to set up with a speaker. As for a portable MP3 player, I think that can work as well if you can find one small enough to fit (and surely there should be something out there tiny enough with not a lot of memory since your average thumb drive isn't too big, yet contains a lot of stuff on it).