Since the dawn of.... WBC1, Wisps and Ancient Wisps have never had any "build" or "cantbuild" speeches. Although Joe the Bartender mentioned that his latest version of the game contains new speech files for such units, I'm not willing to download the entire thing just to see what he's used.
So, instead I've made my own using some pretty basic but effective cropping to give both the Wisp and Ancient Wisp their own build and cantbuild speech files. These can be found in the usual place - within the audio section.