Once it's in STL then it's run through a slicer then a GCode to wav converter, see instructions.peachyprinter.com (one of the later steps, it's easier to find in the single page instructions). There were problems with the original direct printing from blender (I think to do with access to soundcard) so the team are making a seperate API and then it can be plugged into any environment like this and I think they're planning on getting printing straight from blender back in with their (better) methodology.