If you are a sworn fan of Harry Potter you can not be that you never heard about this fact.
According to Rowling, the 't' in "Voldemort" is silent, as it is in the French word for death, "mort". Jim Dale pronounced it so in the first four US audiobooks, yet the characters in the film version of Harry Potter and the Philosopher's Stone pronounced the "t"—after which Dale changed his audiobook pronunciation accordingly.
source: https://en.m.wikipedia.org/wiki/Lord_Voldemort
