It's not, sadly. It's embedded somehow in the game's files--the lipsync. And the head-bobbing is updated to fluent lipsyncing for a better visual of the portrait. It makes it less cartoon-y, since Reforged is no longer "cartoon-y" and low-poly. I don't think games these days, even remasters, would stick to all old methods of speaking, or games in general with dialogue lines. Even in Heroes of the Storm, they tried to make perfect lipsync, but they stopped due to budget cuts. And Anduin, amongst other newer heroes, don't have a proper mouth lipsync in the game (HotS). His voice lines are said, but his mouth isn't opening at all, like his mouth mesh or whatever it's called is detached to prevent animation for it when he tries to "speak". An example of a non-fluid lipsync or mouthing in Heroes is Anduin's Neo-Vigilante skin. That skin now has mouth movements, but it follows the generic cartoon-y dubbing of the mouth you speak of, like Anime shows and films, where the mouth matching words being said is disregard, and only the length of how long the head-bobbing is based on the length of the voice line. Just like the original Warcraft III.
And yes, if Blizzard made the World Editor less limiting, we could just be adding new folders for new custom sound sets rather than replacing existing ones. Or we could have mixed and mashed data values when making spells and abilities rather than coding them via triggers. There's just a lot of issues here that existed even in classic Warcraft III. It's just restricting us.