I am testing myself to make a 4 button keyboard, but I keep having a problem when I add another tone and button. I think it is a programming issue because when I comment out the code for the second tone the frequency does not increase.
Take a look at my piano code here:
http://forum.arduino.cc/index.php?topic=179761.0
I'm definitely going to try yours. I figured out the problem with mine, I have two speakers and I accidentally assigned one to the wrong pin.
Listening to non-playing outputs would be kind of quiet.