No one knows. No one has found a pronunciation guide from ancient Greece (and even if we did, it would be written in a language we didn't know how to pronounce, so it wouldn't help) and obviously we don't have audio recordings.

I've taken a few graduate classes in Classical philosophy, and in each the professor said that we don't have the correct pronunciation of terms, so we should just say it phonetically or however we prefer.

What matters is what Thomas More intended in the 16th century, which other comments speak to.