Why the Monster does not speak
I see that many of the user reviews wonder why the Monster does not speak in this film. After all, he did speak in the previous movie, "Bride of Frankenstein."
The answer seemed clear to me. The Monster was sick, and Wolf Frankenstein tried to bring him back to full health. He was not immediately successful and was surprised when the Monster was able to walk. However, he tells Ygor that the Monster is not fully recovered and that the brain is not yet well.
So it seems obvious to me that the Monster was still a little sick and had lost his power of speech. He also lost the empathy he had in the previous movies.
That's the in-the-movie rationale, of course. I'm sure the *real* reason is that Karloff (who objected to speaking in the previous movie) and the screenwriter wanted the Monster to be more elemental and less complex. So the speech and empathy went away, and they came up with an explanation to give in the movie.