Hey ericlewis - this is cool! Can I ask how you figured it out?!
Doing this for my PO-33 was on my bucket list.
I thought I might have to resort to voltage glitching to get a firmware dump, which is currently beyond my skills.
It's sending sound parameter information (e.g. filter cutoff at 12khz, resonance at 1.6, Q of 0.89) along with note information (e.g. start playing note A4 with velocity 80). You could absolutely use a MIDI CC channel to convey this information. The OP-32 chooses a different route and encodes this into an audio signal so that it can transmit it over the air using the speaker/mic instead of a wireless stack.
Wait, isn't this what MIDI is for?
I bet it sounds like a dial-up tone!
https://youtu.be/TLzAyouLQxw?si=mm6goX-cMyEZkwOI&t=102