See "Chirpy TX/RX": a protocol and implementation for transferring data out of an airgapped device over a piezo buzzer.
Here's some recent discussion of the non-pro version
I really enjoyed the previous iteration of this (https://blog.jgc.org/2022/10/pimping-my-casio-with-oddly-spe...) and I'm backing this one. These are fun upgrades to an iconic watch.
I've been wanting to buy a sensor watch for a while now. I'm actually really glad I waited to back this now!
I would love an SPL meter on my watch with weighting filters! Don't know much about piezo buzzers but I wonder if it could be used as a microphone for this purpose.