There are lots of different types of Self-Playing Instruments, but the folks at Stupid Inventions wanted to create something new so they decided to build the Self Playing Harmonica. Made from an old printer and a vacuum cleaner, the Self-Playing Harmonica will play different songs based on the image you print.
You can learn more about this and find step-by-step instructions for constructing your own self-playing harmonica at Instructibles.
Hehehe. That's a very simple one though; the real challenge is getting the vacuum cleaner to blow as well as suck so that you can get the notes you want - then you have to be able to select blow/suck, the position, and whether you do a glide or stop the air flow as the next note is selected. It would also be nice to get rid of the horrible roar of the vacuum cleaner.
Of course you can eliminate the vacuum cleaner and printer altogether and make use of relatively fast acting proportional valves to drive the harmonica - in which case you end up with a bulky beast which costs the same as a car but you can make it sound just like a talented human was playing it. After all, the harmonica is known as the "mouth organ" and organs got rid of the bellows many decades ago and some will even play themselves (though I don't know of any that are currently built to match a human - I don't know of any machines that tweak the knobs and bars).