I pronounce it, “R’lyeh,” assuming that’s how it’s spelled. “Rye-la,” would be spelled “R’yleh” at best. You’re transposing two sounds.
That is, I would say, “rəl-YEH,” roughly. That said…
According to Wikipedia, it’s [ˈrəʔlʲɛx]. So there’s an official(?) pronunciation(!), there’s a glottal stop, & there’s a sort of fricative “h” at the end. I expected none of that, but yes, you are supposed to pronounce the apostrophe and the h. Who knew?
I always thought it was RYE-la. But my tongue is lamentably non-squamous.
I agree you’re doing it right. These are supposed to be inhuman sounds, so I try to make the sounds in my throat and use my mouth and tongue as little as possible.
Fhtagn is the easy word in that sentence–just like it’s spelled!
BTW, by one of those curious cosmic coincidences, Ken Begg has just put up a picture of Cthulhu from this film on his Jabootu site: Monster of the Day #207 – Jabootu's Bad Movie Dimension
I’ve always said it as the film people in the OP, Ruh-LAY, though I couldn’t tell you where that pronunciation originated with me.
As a side note, there’s an interesting mythos story about a surgeon performing some minor corrections on the vocal apparatus of a woman – nothing big enough that it really should be noticeable, but naturally, she ends up looking all wrong and eldritch and, possibly, non-Euclidean – in order to enable her to pronounce the words correctly. Of course, when said right, it turns out that those fun phrases aren’t really all that gibberish at all…
So all of you unable to get it right probably should be grateful.
Ph’nglui mglw’nafh Cthulhu R’lyeh wgah’nagl fhtagn!
For those of you lacking my lingual abilities, you can say:
Fi-nig-lew-ee mugel-wa-naf kuh-thoo-loo rill-yay wuh-gah nah-gul fa-tag-en.
It won’t matter to Cthulhu how you say it; you’re gonna get et anyway.
Like the state capital of North Carolina.
Url-YAY - The R becomes vowel-like, as in Slavic languages.
Definitely not Rye-leh. That reverses the L and the Y
I always liked Eben Brooks’ pronounciation of it. The sentence is at 2:30, but the whole song is great.
I could tell you, Miss Mapp, but then I would have to kill you. There are some things woman was just not meant to know. 
I had mentally pronounced it “rye-leh” before realizing that would imply the transposition of two letters. I haven’t actually pronounced it out loud, nor do I ever intend to.
I have always pronounced it Ruh ail ee.
As likely the only poster to have worked at Weird Tales, and to have done a one man street show as a preacher of Cthulhu’s gospel, I like to think my opinion has more weight than most.
Even if it doesn’t.
I think it is rrolled “r”, crooked letter, Krill-yea, crooked letter.
But if Cthulhu likes you, he’ll eat you last.
“Rye” was my attempt to pronounce the apostrophe as a short “i.” “La,” I suppose, was meant to be more of a schwa “eh” than an “ah,” but it’s hard to write out these nuances of vowel sounds.
I stand corrected.
First… last… which would be preferred?
I’d say that it’s a good policy to assume that every symbol in a transliteration (even fictional ones) should be pronounced, including apostrophes and Hs. That’s the way transliterations should work, anyway.