Hi, I have the same problem as nsg.
I have two Bangle.js watches (The Both hands pledge) and neither of them has the beep() making any sound.
I have removed the back from one of the Watches and there is no piezo visible and no wires soldered to the pads shown in your photo above - just the battery below the circuit board.
I am assuming the second one is the same.
I have some piezos with a diameter of 2 cm that look around the right size.
Should any piezo that fits be ok? Or should I wait for you to ship me some?
Would they need to be stuck to the battery or insulated from the board or anything?