Puck.js out of stock

I started to develope a solution for disabled people

I think I saw you posted a video of that? I looked great!

if it possible to use Puck.js and the MDBT42Q for a commercial project?

Yes, absolutely! That's the idea - plenty of people do exactly that.

I soldered one between D11 and GND on Puck.js

That link does say 'buzzer'... What happens if you just do D11.set()? Sometimes these things have electronics in them to actually have a sound when you apply power (it's not just a normal speaker).

Otherwise, if it doesn't...

• The docs mention a resonant frequency of 4000Hz - so if you make your beep that frequency it might be louder
  
• Right now you're outputting a square wave between 0 and 3v. It is possible to double that - going from -3v to 3v, by putting the buzzer between two IO pins that are the inverse of each other. It's a bit more involved than just analogWrite, but there's an example here: http://www.espruino.com/NRF52LL#create-a­-square-wave-on-pin-d0-with-the-inverse-­of-the-square-wave-on-d1-