Looks like you have done a lot of work there, if your not using additional external effects for performances you should take a look at the delay code here -
But something that might be more interesting is that I have now added trigger to the oscillators in the five dollar synth, what this means is that I can use the keyboard to trigger the other oscillators leading to something very similar sounding to the Auduino with keyboard pitch control -
The keyboard code needs a complete rewrite but the four oscilator synth engine is now pretty capable.
Duane B