Click a square, then click 2, then... well, I'll leave you to play and work it out* (oh, and if you hit the "space" bar on your keyboard, it clears everything)
I know, I know, some of you will already know about this, indeed will probably have it as an iPhone app or something. So far as I can make out, it was created a couple of years ago. But I only found it today. I wanted to put it on my blog so I would be able to find it again any time I wanted to for the rest of my life.
For reference (and for those reading this on Facebook or their RSS feed), the original can be found here:
While I took this embeddable Flash version from here:
*just wait until you decide to write your name in sound...