Forgive me for bringing this ancient thread back from the dead ... however, the correct pronunciation of this creature has been much on my mind lately.
From what I've been able to find online, the proper Greek-ish pronunciation is like "eh-rih-NEE-ess", while the Anglicized pronunciation is "eh-rih-NEE-eez".
That being said, what I don't get is why WotC turned the plural form into a collective noun. What's wrong with "erinys" as a singular?
On a similar note, it would seem they've also collectivized manes demons, so a single one is still a "manes" not a "mane". Weird, eh?