@fotofieber DC+ one output is really poor and voltage has no impact on noise. I have tested with up to 6S LiPo and there's absolutely no gain as compared to 5V DC
AC+ two outputs is a game changer! Much louder right from +/-5V and really a killer @ +/-12V.
So that's it: the first iteration of this project will go for a step-up converter from 5V up to 12V for the piezo speaker.
EDIT: could someone suggest a MOSFET (or transistor) that could be used in lieu of the huge (and probably over-sized) TIP120 (that where there, waiting in the drawer ). Through-hole is my league. SMD is a bit of a struggle (welding, machining pcb, ...)