Hi,
While cleaning my bookmarks, I stumbled on this again…
http://3dmin.github.io/
Replace the X-OSC or Bella with something (arduino?) able to generate MIDI or CVs/Gates/Triggers,
and… pump-it up 
The build is “slightly” more involved than a soft-pot, sadly…
Oh nice !
I have a small accordion but it’s out of tune and i try to figured out how to convert it to midi.
I still have to firgured how to know if i push or i pull, and how to detect velocity
Looks like they are using valves, one in each direction.
http://3dmin.github.io/build/valves.html
And microphones to detect the air flow.
Yep but i dont qute understand how they use the microphone, if it’s just as a push-pull sensor or as a velocity sensor i guess i should take a look at the firmware
I didn’t analyse it too much, too complicated for me to build
But I guess an average on the “volume” of a microphone will give a decent approximation to the speed of the “wind”.
There’s also this
Googling “midi concertina” also turns up some stuff but mostly just keyboards without any bellows sensor (and often without any bellows at all).