Regarding "silent" protagonists in player-to-player dialogs (single-player). I've checked a couple of other dialogues (just got into Tutorial Dungeon).

So I rolled two characters BOTH with AI Personalities. Now the effect is two-sided. No matter which of main characters starts the conversation, his/her lines are mute, and only the other one's are voiced.

I will revert to complete No AI option for the moment, because I want to hear all the new voiceover.

Thanks for your swift replies Raze!