Holy shit, this looks fantastic. So glad to see they're doing this one as a talkie, too, since it's a story that would absolutely not work without sound...
As I understand it, the concept is that this has sound because the original story was written after the advent of talking pictures, whereas Call of Cthulhu was written before that. So, for future productions the idea is that the films are meant to represent what it would have been like if the film had been made at the same time of publication.