Sometimes people will talk without opening mouths or no voiceover at all

maboleth

At first I thought it was Lyris during prologue quest as it happened 2x already, but it happened for the 3rd time too in Western Skyrim. During playing, random NPCs will just keep their mouth shut while voiceovers will be heard (Jur, the sailor guy is the latest example). Sometimes not even that (mouth shut + no voice).

Current remedy is to logout/in again.

Lannharr

Same, it happened with a couple of NPCs. Part of the dialogue was without mouth movement but voice.

Cheveyo

I had one NPC start talking fine but then it was like their volume rapidly turned down as they were talking.