I think Hadrhune first appeared in Troy Denning's Return of the Archwizards trilogy. He was a significant character in Paul S. Kemp's Twilight War trilogy and, I think, The Godborn (second novel in The Sundering series). You could put the question to these authors. You might perhaps instead obtain audiobook versions of these novels, although I'm unsure if the narrator's pronunciation would necessarily agree with WotC's "official canon".