Karrnathi zombies and Skeletons from the Eberron Campaign setting have an intelligence score and can speak. Granted ther eis noting stating how these undead are created.
Awakening. I don't know exactly what this does. Where is this found?
EDIT: Ah OK so an awakened undead has an intelligence of 7-10. Definitely a sufficient intelligence to speak. Does the spell say anything about speaking, or if they gain skill points? If so, I would throw one point into Speak Language and say, yes, the undead can speak. It may not be all that intelligible, but look at some of the people who speak today and you can't understand a word they are saying, even though they are speaking your language.