Skyrim was running great until the last update installed by steam. All sounds are ok but the voices are missing. All NPCs just look at me with a poker face for some time waiting until the dialog line ends.

I reinstalled the game and updated all my drivers but none of that worked. Any idea how to fix this?

Use http://skyrim.nexusmods.com/mods/4804 to unpack Skyrim - Voices.bsa in your Data folder.