Have you ever thought, that the Voice of the Emperor, could actually be the same voice as the Dovahkiin's? All Emperors up to Mede Dynasty have been dragonborn, and perhaps they may have had some minor voice skills, who their people could use to an even more inferior degree (Only once per day, not like once per minute as it is with most shouts, to my knowledge). Of course, the term Thu'um was first introduced in Skyrim, but who knows for how long bethesda had that planned? The term of dragonborn emperors, and having the dragon blood in ones veins, has existed for a long time now.
Now my question is this: Now that the Empire does not have a Dovahkiin Emperor anymore, should the Imperials still be able to tap into the powers of their former Dragonborn Overlords? Tecnically, no. If the power has any connection to the Storm Voice (Thu'um), then actually Imperials shouldn't be able to do so.
Post your opinions!