I've added a 8ohm 0.5W speaker, its a bit large, but it fits in approximately the same space as the ProMicro. I soldered to the pcb hole labeled PC6 on my Rev 3 Planck PCB, and GND on the near by chip. [Pic of an identical mod][https://i1.sndcdn.com/artworks-000128070066-myh0ls-t500x500.jpg] (not mine).
![https://i1.sndcdn.com/artworks-000128070066-myh0ls-t500x500.jpg]](https://i1.sndcdn.com/artworks-000128070066-myh0ls-t500x500.jpg%5D){kind=link}