From my blog, Language Hippie:
Language diversity and tolerance might not seem to immediately apply to the adventures of a young boy wizard, but I think the [Harry Potter] series can actually teach us a lot. Anyone who has both read the books and watched the movies has surely noticed some differences between the two. In addition to details that were changed in the transition from novel to film, there are also elements that were unspecified in the original stories and had to be filled in by the moviemakers: from the tune of the song the mermaids sing, to the way werewolves transform at the full moon, to the exact design of Voldemort’s wand. Most likely, readers imagined such details quite differently in their own minds, before witnessing how they were presented on film. As a result, the story we watch is somewhat different from the one we had experienced before.
This, I believe, is a fairly obvious point that has probably occured to all of you before now. And it is my hope that you will also accept as noncontroversial my further claim, that the version of events and details presented in the Harry Potter movies is not the only legitimate one. That is, I hope you will not consider yourself to be incorrect in how you imagined the stories before, just because the filmmakers viewed and presented them differently. While their vision may have the original creator J.K. Rowling’s seal of approval, and yours likely does not, that does not mean that your own version of the story is in any way misguided or wrong.
I mention all of this as set-up for my ultimate point, which is about the language of the Harry Potter books. This is another element of the novels that had to be filled in by the filmmakers, either with or without consultation by J.K. Rowling. (I am guessing the former, but I consider the matter to be ultimately moot.) Although Rowling’s books are full of unusual character and creature names, magical spells, and so forth, she usually provides little indication beyond spelling as to how these words are to be pronounced. And so, when those words are spoken aloud in the movies — or the audiobooks, for that matter — the producers must decide on one interpretation, one vision, to present to the audience. And like any other detail, those pronunciations might not agree with how we imagined the world of Harry Potter ourselves.
I urge you, then, to not be swayed by how the movie characters pronounce J.K. Rowling’s magical words, if you have always believed in your heart that they were said differently. If you think ‘accio’ should have a ‘CH’ sound and not a ‘K’, if you rhyme ‘muggle’ with ‘frugal’, or if you pronounce ‘Voldemort’ with a silent -t, don’t be discouraged that the people in the movie franchise do things differently. Theirs is but one interpretation, one dialect, of the words J.K. Rowling has gifted to us. Your own is just as valid, and just as worthy of celebration.