Try my piano project here. Coded to play 13 notes from 13 buttons on a '1284P (which has 32 IO). Pretty well commented, really just a big expansion of Blink Without Delay for 13 outputs.
http://forum.arduino.cc/index.php?topic=179761.0
Youtube video of me playing 8 notes (run out of buttons)
The 13 outputs go thru a some resistors and caps to be mixed analog-wise, then played thru a self powered computer speaker, schematic is shown at the topic page also.