I should start by acknowledging that those of you who have heard about this phenomenon will be disappointed with the answer I will provide, or lack thereof. In the Torah (and this phenomenon seems to occur only in the Torah) in all but 5 places, the pronoun which הִוא means she is spelled with a vav (purists will prefer the representation waw, ו) rather than a yod (י, הִיא) which is the way it is spelled those 5 times and everywhere else in the Bible and after the Bible.
In an effort to pronounce what they see, I have occasionally heard people say “heeve” when they come across the word in the Torah. We know from our Masoretic tradition that this is incorrect. It should be pronounced “hee” just as if it were spelled the more usual way (at least outside the Torah).
It looks as if the author(s) of the Torah pronounced the word “hoo” regardless of whether the pronoun referred to “he” or “she” (I’m not claiming that this is how they actually pronounced the word, only that it was spelled that way.) I am not aware of any tradition that suggests we should pronounce it “hoo” when it refers to “she.”
How do I know that the Masoretic tradition demands the pronunciation “hee”? Well, that’s what the vowel point is that is included in all the Masoretic manuscripts. The Masoretes never change a consonant in the text (although they may add variants), but they use the vowel pointing system to teach us how they believe the word should be pronounced.
One more pronunciation note to those who might be tempted to pronounce this word “heev”. Originally (and by originally I mean in all periods of classical Hebrew) the vav was pronounced “w”–therefore scholars occasionally refer to the letter as “waw” rather than the “vav” of later Hebrew. A trailing “w” will always have a faint pronunciation because of the phonetics of the letter. So at best we are speaking of the difference between “hoow” and “heew”.
Getting back to the accepted traditional pronunciation (“hee” rather than “hoo”), this is a particular case of the more general phenomeon of Q’ri and K’tiv. The Q’ri (what should be read out loud) is “hee”, the K’tiv (what is written) is “hoo”. At a future time I hope to return in greater detail to a discussion of the Q’ri-K’tiv issue.
Ultimately most Hebrew students want to know “Why this is?” Why should “hee” be written “hoo” and why do we see this phenomenon only in the Torah. That is the disappointment I mentioned at the beginning of this article. No one knows. At least I have never heard a theory that satisfactorily explains this. If you have, perhaps you’ll comment here.