From the Watchtower's own website:
http://www.watchtower.org/e/na/index.htm
How Is God's Name Pronounced?
The truth is, nobody knows for sure how the name of God was originally pronounced. Why not? Well, the first language used in writing the Bible was Hebrew, and when the Hebrew language was written down, the writers wrote only consonants—not vowels. Hence, when the inspired writers wrote God's name, they naturally did the same thing and wrote only the consonants.
While ancient Hebrew was an everyday spoken language, this presented no problem. The pronunciation of the Name was familiar to the Israelites and when they saw it in writing they supplied the vowels without thinking (just as, for an English reader, the abbreviation "Ltd." represents "Limited" and "bldg." represents "building").
Two things happened to change this situation. First, a superstitious idea arose among the Jews that it was wrong to say the divine name out loud; so when they came to it in their Bible reading they uttered the Hebrew word 'Adho·nai' ("Sovereign Lord"). Further, as time went by, the ancient Hebrew language itself ceased to be spoken in everyday conversation, and in this way the original Hebrew pronunciation of God's name was eventually forgotten.
In order to ensure that the pronunciation of the Hebrew language as a whole would not be lost, Jewish scholars of the second half of the first millennium C.E. invented a system of points to represent the missing vowels, and they placed these around the consonants in the Hebrew Bible. Thus, both vowels and consonants were written down, and the pronunciation as it was at that time was preserved.
When it came to God's name, instead of putting the proper vowel signs around it, in most cases they put other vowel signs to remind the reader that he should say 'Adho·nai'. From this came the spelling Iehouah, and, eventually, Jehovah became the accepted pronunciation of the divine name in English. This retains the essential elements of God's name from the Hebrew original.
How Is God's Name Pronounced?
The truth is, nobody knows for sure how the name of God was originally pronounced. Why not? Well, the first language used in writing the Bible was Hebrew, and when the Hebrew language was written down, the writers wrote only consonants—not vowels. Hence, when the inspired writers wrote God's name, they naturally did the same thing and wrote only the consonants.
While ancient Hebrew was an everyday spoken language, this presented no problem. The pronunciation of the Name was familiar to the Israelites and when they saw it in writing they supplied the vowels without thinking (just as, for an English reader, the abbreviation "Ltd." represents "Limited" and "bldg." represents "building").
Two things happened to change this situation. First, a superstitious idea arose among the Jews that it was wrong to say the divine name out loud; so when they came to it in their Bible reading they uttered the Hebrew word 'Adho·nai' ("Sovereign Lord"). Further, as time went by, the ancient Hebrew language itself ceased to be spoken in everyday conversation, and in this way the original Hebrew pronunciation of God's name was eventually forgotten.
In order to ensure that the pronunciation of the Hebrew language as a whole would not be lost, Jewish scholars of the second half of the first millennium C.E. invented a system of points to represent the missing vowels, and they placed these around the consonants in the Hebrew Bible. Thus, both vowels and consonants were written down, and the pronunciation as it was at that time was preserved.
When it came to God's name, instead of putting the proper vowel signs around it, in most cases they put other vowel signs to remind the reader that he should say 'Adho·nai'. From this came the spelling Iehouah, and, eventually, Jehovah became the accepted pronunciation of the divine name in English. This retains the essential elements of God's name from the Hebrew original.