There are some letters that are hard to pronounce even by the most seasoned ventriloquists. What they will do is they will make a sound that is most similar to that letter especially if you are familiar with the word that is stated by the ventriloquist. Your brain will then process it as the actual letter even if it isn’t.
This is a trick of association. B, F, P, M, and V are considered to be tricky letters to say. W is also considered to be a problematic letter but not as hard as the other letters that are mentioned above. Some ventriloquists take time before they become experts at what they do. Constant practice is the key.