I won’t pretend I wouldn’t consider it justified if Blizzard did decide to recast Kael’thas over accusations of him grooming young fans, but I have no idea and no evidence of that - there’s literally nothing available so far as to why these changes were made. There’s speculation based on the recent allegations of sexual misconduct leveled at Flynn, but there’s been no statement from either Flynn himself or Blizzard about what’s going on with Kael’thas’ voice, and why it was changed. We also do not know why Quinton Flynn has been replaced.

We do not currently know the actor who has replaced Quinton Flynn, who has voiced Kael’thas since Warcraft 3. At present, we also know that all of Kael’thas’ lines in World of Warcraft - including his original The Burning Crusade performance - have been replaced but at present it does not appear that his voice from Heroes of the Storm has changed. The video was by Simaia on Twitter, who has been doing various other datamining and exploring posts, and it’s worth a listen. The folks at Wowhead have a video comparing the original voice lines to the new ones, and it’s subtle but definitely noticeable that in patch 9.1, Kael’thas Sunstrider is now being played by a new voice actor.

Heroes of the storm sound files