These sound like: ''th" . As in English ''this'' ''those'' . In Greek is the letter δέλτα (delta) or (thelta). I hope to help you.

The delta at the beginning is supposed to sound like "th" as in English "this, that, though".

That sound and the "v" sound can sound similar as they both involve the teeth -- one is labiodental (lip against teeth = v), the other dental (tongue against/between teeth = th) -- and are both voiced.

But they're different.