I've been playing around with how speech and voice files and voice acting scripts work in AGS.
What I've discovered is that option text within dialogues is always assigned to the character which was marked as the 'player character' for the beginning of the game (or perhaps more specifically the character marked as the 'player character' when the voice acting script is compiled?)
So, if a game changes player (either as a genuine part of the story or because I've used a placeholder character for menu screens/cutscenes etc) then the correct voice file will never play when a dialogue option is selected.
The way round this I can think of is to untick all the 'say' boxes and just manually code at the beginning of each dialogue option which character should say it.
But I thought I'd check before starting to do that - is there something I'm missing here?