JK Rowling has said the Voldemort is pronounced Voldemor (with a slient t). In the movies they always pronounce it Voldemort.
With what little I know about the French language, I know that Beauxbatons should be bo-ba-to (French).
Despite failing at French, they got the Celtic names pretty well. I was actually surprised when Hermione and Seamus were pronounced correctly.
If you count spells, there's also Accio.
The letter C in English is pronounced like K when coming before an A, O, U, or another consonant, and like S when preceding an E, I, or Y. (Examples: cart, coming, curt, preceding, civil, fancy, includes.) This includes when two C's are adjacent, as in the word accent - pronounced aksent because the first C comes before a consonant while the second precedes an E. Therefore the word Accio should be pronounced aksio.
This is borne out by Stephen Fry's audiobook versions of the Harry Potter books, for which he consulted heavily with JK Rowling, and in which Accio is indeed pronounced Aksio. But in the films it's pronounced Akkio, which sounds quite different.