Fully voicing all the dialogue of every single NPC would probably be too expensive, but SE could certainly add some generic phrases when you click on an NPC, such as "Hello!" or "Good day!" And when you close the dialogue window, the NPC could say another phrase, such as "Good bye!" or "Twelve be good!" or "Keep your feet on the ground!"

The actual dialogue itself wouldn't have to be voiced, but it would be nice to have a few simple greeting phrases like other MMOs.