If I'm recalling the HoME books correctly, somewhere, Tolkien said that originally, none of the Ainur had names among themselves, because they spoke from mind to mind and thus recognized one another without the need for names as we think of them. That would make perfect sense, since language and speech is a convention of the physical world. While words were the delight of Manwe, I doubt very much that it was a delight he had until he entered Ea; language was the chief characteristic of an incarnate being, and until they themselves incarnated within the physical world, the Valar would have had no need for it. The Valar made their own language before the awakening of the Elves in anticipation their coming -- and even then, the Elves didn't care much for the sound of Valarin. Manawenuz became Manwe to the Elves, Ulluboz became Ulmo, Ibrīnišilpathānezel became Telperion, etc. (sorry for the lack of special characters; I still don't know how to do them via the html keyboard
). If they made names for themselves during this period, Sauron's first spoken name would have been in Valarin, but I don't believe it's ever mentioned; not a whole lot of Valarin words are known. Especially if it was a name he chose for himself, this would have been Sauron's "truest" spoken name, I should think, but very few people outside the Ainur might know what it was.
Whether or not any of his servants or slaves actually DID know his true name would not prevent Sauron from issuing orders against it being spoken or written. That he could not imagine anyone wanting to destroy the Ring shows that he had the kind of twisted mind that always believes his enemies are planning to do what he himself would do. He would think it necessary to prevent his name from being used against him because once he had his Ring and his power back, he would certainly use any knowledge of his enemies' true names against them.
Y'know, I think I know WAY too many people with that kind of dysfunctional mindset.... *sigh*