There's a reason he was known as "He Who Must Not Be Named": He's sick and tired of everyone saying his name incorrectly. J.K. Rowling confirmed that we have all been saying Voldemort's name the wrong way. Considering that this has been happening since 1997, it's all extremely awkward. The "T" at the end of the evil one's name is silent — think The Colbert Report, like "Voldemore." So if you've been leaning heavily on the last letter, you've been saying his name wrong. Though frankly, he kind of deserves it, being so evil and all.
Rowling doesn't shy away from blowing our Muggle minds on Twitter, and she revealed this tidbit of information during a round of answering some Harry Potter trivia. She stated the correct pronunciation, but then admitted that she's likely the only one who uses it. So if you have been laughing at Jim Dale's (correct) pronunciation of "Voldemort" on the U.S. audiobooks, then the joke is on you.
Rowling has always put a lot of thought and care into naming her characters, and clearly, Voldemort received that same level of attention. Draco Malfoy's first name, for example, is a reference to "Draconian," meaning "unusually severe or cruel." And the root of "Malfoy" is "mal," meaning "bad" or "evil," and "foy," generally the root of "faith." (Uh, yeah.) Then there's Remus Lupin and the whole "Lupin"/"lupine"/"wolf" giveaway. And Minerva McGonagall? Minerva is the name of the Roman goddess of wisdom.
So where does Voldemort (silent T) come from?
For those of you who paid attention in high school French class, you know that "Mort" (with the silent T) is French for "death."
Oh, J.K. You're always so on point.