God isn't very good at preserving his name, not like one would expect from an omnipotent deity who prides himself so much in his name. So we don't really know how it was originally pronounced in Hebrew.
Yahweh is the word that most researchers believe to be closest to the original pronounciation. So if you ran into an ancient israelite and you wanted to talk to them about their god, your best bet is to say 'Yahweh'. With a sort of V sound on the W.
Like a cross between Yahweh and Yakvey. Even the Watchtower admits that it's the most accurate. (Although they try to play it down as much as possible)
Basically they use Jehovah because and ONLY because it's more familiar to people. Which is a pretty dumb reason to use the wrong name. They're really just afraid that people with think they're pagans if they start using a name nobody is familiar with. Although even that reason, which was lousy to begin with, is pretty weak considering that most people nowadays are familiar with Yahweh perhaps even moreso than Jehovah.
From the insight book under 'Jehovah':
What is the proper pronunciation of God’s name?
In the second half of the first millennium C.E., Jewish scholars introduced a system of points to represent the missing vowels in the consonantal Hebrew text. When it came to God’s name, instead of inserting the proper vowel signs for it, they put other vowel signs to remind the reader that he should say 'Adho·nai′ (meaning “Sovereign Lord”) or 'Elo·him′ (meaning “God”).
The Codex Leningrad B 19A, of the 11th century C.E., vowel points the Tetragrammaton to read Yehwah′, Yehwih′, and Yeho·wah′. Ginsburg’s edition of the Masoretic text vowel points the divine name to read Yeho·wah′. (Ge 3:14, ftn) Hebrew scholars generally favor “Yahweh” as the most likely pronunciation. They point out that the abbreviated form of the name is Yah (Jah in the Latinized form), as at Psalm 89:8 and in the expression Ha·lelu-Yah′ (meaning “Praise Jah, you people!”). (Ps 104:35; 150:1, 6) Also, the forms Yehoh′, Yoh, Yah, and Ya′hu, found in the Hebrew spelling of the names Jehoshaphat, Joshaphat, Shephatiah, and others, can all be derived from Yahweh. Greek transliterations of the name by early Christian writers point in a somewhat similar direction with spellings such as I·a·be′ and I·a·ou·e′, which, as pronounced in Greek, resemble Yahweh. Still, there is by no means unanimity among scholars on the subject, some favoring yet other pronunciations, such as “Yahuwa,” “Yahuah,” or “Yehuah.”
Since certainty of pronunciation is not now attainable, there seems to be no reason for abandoning in English the well-known form “Jehovah” in favor of some other suggested pronunciation. If such a change were made, then, to be consistent, changes should be made in the spelling and pronunciation of a host of other names found in the Scriptures: Jeremiah would be changed to Yir·meyah′, Isaiah would become Yesha?·ya′hu, and Jesus would be either Yehoh·shu′a? (as in Hebrew) or I·e·sous′ (as in Greek). The purpose of words is to transmit thoughts; in English the name Jehovah identifies the true God, transmitting this thought more satisfactorily today than any of the suggested substitutes.
So if we're not ABSOLUTELY certain that your name is pronounced Jason, we may as well just keep calling you Gerald. That's the respectful thing to do right?