Their own encyclopedia - Insight into the holy scriptures admits that the original pronunciation of the name is lost and that they use the common English version under the heading 'Jehovah'. (if it hasn't been changed by now)
How do you base a whole multi million member organisation (that control peoples lives, to the point of having them die or breaking up families) if you don't even know the real name of your God? This is especially mind boggling considering the emphasis they put on the importance of said name!