I'm not sure what you're talking about. I've been playing with the Red Prince and he's always talking whenever he enters a room, or new place, etc. It is fairly common.
As for dialogue with NPC, it would probably slow things down too much? (plus a ton of extra recording) Consider that, currently, once you pick a dialogue option, the NPC immediately replies to you, basically taking just about the same time to hear the voice line as it would take you to read it on the panel.
What you are suggesting would be a bit redundant. You read a line, choose it, then have to wait while listening to the audio line for the option you already read. After a couple of hours most players would just skip the audio line entirely, making the whole investment in audio for those lines pointless.