I got it from this thread - also linked in the first post of this thread. You actually helped that individual with some of their requested custom features.
It's a variant of Dean's first sketch that was a Piezo/drum pad-based sketch. I was unable to get Dean's original piezo based sketch to work, but I was able to get this variant to work without false triggered notes (with some minor modifications - most notably I had to use Dean's original int offsets arrays).
The Neopixel aspects of the code I got from here, and it is a variant of Dean's other softpot guitar sketch that uses force sensitive resistors instead of Piezos. I would love to have just used this one, but with this one - I could not get notes to turn off - even when setup exactly as his wiring diagram shows, with all resistors in place. I was unable to get Dean's original FSR-based softpot guitar sketch to work because I could not able to figure out how to substitute the Adafruit Trellis with functional switches.
As I mentioned in posts #20 and #21, both of the sketches are variants of Dean Miller's two sketches.
I have already learned a lot from this thread, and I hope to simplify the sketch so that it can run much more efficiently! With that said, if you think it would be easier to troubleshoot one of the above sketches (and have neopixels) instead of working with this atrocity of a sketch - then I would be all for giving that a try too.
Please forgive my technical ignorance, but I thought drum pads were basically piezo transuders/elements. I've made a few MIDI drum projects with arduino using piezo elements. Am I wrong?
Yes I've been fiddling with this all day and will be posting my results shortly. Hopefully it'll at least be closer to the info you need this time. Thanks again for your guidance!