I'm not certain when it started happening but I guess after the 1.6 patch. But now I've noticed several NPCs suddenly lost some audio lines when I'm conversing with them. 2 NPCs that do or did (past tense because quest is over with.) Is Sybille Stentor giving me the "kill the vampires" quest she is mute while the dialogue lines appear.
And the other was Odahviing explaining his deal with the Dovahkiin while in Whiterun. They are just brief segments of dialogue that are just missing. I don't know why, and it could also be a mod that has caused this but I have no clue what.

I'm guessing I should extract the voices bsa files and put them in the sounds file to fix this but I really don't want to lose a 1 gb of space to something that is compressed in the bsa files.

It sounds like it could be the dialogue bug that was introduced by the v1.6.89 patch. Dialogue that is edited by a ESP or ESM may not play at all and has been known to break quests.

At the moment there isn't a way around this bug except to remove any mods you have that you know to edit anything to do with dialogue, roll back to v1.5 of the game if you backed it up, or wait for a fix that may or may not come in a later patch.