I wasn't even aware that any delta firmware could command motors directly.
So using your firmware the deltas can play polyphonic sound, too bad I have only Mega2650, I soo want to try that.
And I like your idea about using extruder motor for the second channel.
Here is another one - what if 10 people get together and make 10 deltas play a symphony
All about delta 3d printers in one place [
www.deltarap.org]