The pronunciation wasn't really lost. The Hebrew alphabet does use some of its consonants as helper vowels. Josephus says the tetragrammaton was written with four "vowels" not with any four letters or four consonants.
Look at the word for Judah, which is Yehudah in Hebrew. It's spelled Yod-Heh-Waw-Dalet-Heh and the pronunciation sounds like Yay-hoo-DAH with the emphasis on the last syllable.
If you take the letter Dalet out you're left with Yay-hoo-AH with emphasis on the last syllable. And if you take the Dalet out you have the letters Yod-Heh-Waw-Heh left which is the spelling of God's personal name.
Now Yay-hoo-AH is very close to how it is pronounced in Arabic today, Ya-hoo-ah, and spelled with the same four letters in the Arabic alphabet which is derived from the same source as the Hebrew alphabet.
Now the W letter has changed to V in many languages, and it did in Hebrew where David is no longer pronounced Da-weed, and in Arabic it's Daoud, but in Modern Hebrew David sounds like Da-veed, much like a Spanish pronunciation of David.
So from Yah-hoo-AH to Yay-hov-AH as the change goes and in Latinized transliteration (from some medieval monk) you get IEHOVA, which came to English as Jehovah and Spanish as Jehová (Hay-oh-VAH).
Okay, enough linguistics, but anyway that's my rationalization of it.