In a letter to the amateur writer Duane W. Rimel, dated July 23, 1934, H.P. Lovecraft addresses the issue of how to pronounce the name “Cthulhu.”
The name of the hellish entity was invented by beings whose vocal organs were not like man’s, hence it has no relation to the human speech equipment. The syllables were determined by a physiological equipment wholly unlike ours, hence could never be uttered perfectly by human throats … The actual sound — as nearly as any human organs could imitate it or human letters record it — may be taken as something like Khlûl’-hloo, with the first syllable pronounced gutturally and very thickly. The u is about like that infull; and the first syllable is not unlike klul in sound, hence the hrepresents the guttural thickness.
Chaosium thus has a lot to answer for on this score, as a great many people now believe, thanks to their RPG, that the name is correctly pronounced “Ka-thul-hoo” or some variation thereof. Maybe that’s close enough for government work, I don’t know, but S.T. Joshi records that Donald Wandrei once pronounced the name as Chaosium does in HPL’s presence “and received nothing but a blank stare in return.” Personally, I think mispronouncing Cthulhu is more forgivable than mispronouncing Conan or Tarzan, given Lovecraft’s own statement that the name was never meant to be spoken by human tongues, but it’s a mistake nonetheless.