I have an interior space with 14 NPC's in it. Between quest stages 36 and 46, one of the characters walks to the center of the room and delivers a speech to the crowd. It worked fine when I tested it before adding the crowd, but now that I've added the 13 other NPC's they keep having random conversations with each other while the main NPC is delivering the speech, and they are drowning him out. How do I make the 13 guests shut up during those quest stages and listen to the speech that they supposedly came there to here?

There is a tweak in bashed patch Tweak Settings -> AI: Conversation Chance

Streamline can also handle that based on current FPS situation IIRC.