It is sometimes noted that we really don't know how the Hebrews first pronounced the divine Name. Was it Yohwih, Yuhiweh, Yahweh, Yahovah or something else? There are complex explanations along the lines of the interchangeable Yahweh and Adonai and the sharing of their sounds.
But today I want to ask (as I am sure it must have been asked by others), can the Septuagint not help us? It was translated in the mid third century before Christ and does include vowels. True, we have few extant manuscripts dating close to this time, but there are a few before the common era. From what we know of Hebrew and Greek translations at this period, especially since the Dead Sea scroll discoveries, is that there was considerable flux in times prior to text standardisations. This would likely have included variations around pronunciation for some names. With all of that in mind, we can look to the best critical editions of the Septuagint for clues on pronunciation.
Take for example the name Joel, an Old Testament prophet. Pronunciation of the Greek gives something like Yo el, and it comes from Yahweh is God. Here, Yo seems to stand for Yahweh and El for God. In contrast, in the Psalms (and also in Revelation), we have Allelujah! Which is the Greek transcription of praise be to Yahh, the contracted Hebrew form of Yahweh.
The latter example seems to suggest that the first vowel of the divine Name, before it was rendered "unpronouncable", might indeed have been "aa". But if that is true, where does the "oo" sound come from in JoEl?