If it his his mother, she'd probably be doing it by using that same imprint thing that allowed her to talk to Harry through Thomas in Blood Rites. if it's Elaine, probably through their mental connection, but amplified. If it's Lash, probably from inside his head. If it's lea (i think this least likely, but hey) well, she probably has some crazy fae magic that lets her do that. I have a feeling that it'll happen again before we figure it out though
Have we heard this voice before, like we did in Changes? I don't remember it before. . .