Completed code, pic of prototype, schematic.
(cap on the power supply rail didn't make it onto the schematic tho, nor did the FTDI module off to the side of the 1284 board)
I connected to an amplified computer speaker. The sounds do not interfere with each other at all. I only had 8 switches, so I connected c-d-e-f-g-a-b-c. Tried pressing up to all 8 at once, did not hear any frequency fluctuation at all. C4 and C5 sound good together, no "beat frequency at all, and also sounds very in tune with wife's baby grand piano.
piano13keyBurstFix.ino (11.2 KB)