Why give Abraham a rough breathing? (Dirk Jongkind)
Isaiah: rough or smooth? (Peter Williams)
The takeaway is that in the Greek manuscript tradition the name Abraham sometimes has a rough breathing and the name Isaiah always (in the manuscripts consulted) does. That means that in the mind of the scribe, both were pronounced with an initial aspiration or "h" sound. A smooth breathing would be silent.
I would not expect this result from the Hebrew forms of the names, but the Greek scribes probably didn't know Hebrew. Who knows where they got the idea? And who knows how the names were actually pronounced in Greek when the Septuagint and the New Testament were written? But, as Dirk Jongkind observes, modern editions have to include a breathing for any Greek word that begins with a vowel. The manuscripts can at least offer some guidelines on which to use.
Visit PaleoJudaica daily for the latest news on ancient Judaism and the biblical world.