Pronunciation of Σίγμα and Ζήτα.

[Windrammer]

How does one produce these sounds? They're not quite like sibilants found in most Euro languages.

[D_..]

It's very easy!

You should consider all σ/σσ/ς letters as "ss" . It is true that in the end of a word, ς, especially when preceded by ο, may sound a bit thicker and might be associated with a sh sound; it's not though!!

A ζ is always a z sound. It might be lighter as in zebra with certain vowels (think ρύζι), or thicker as in measure (ζουζούνι) but it is always a z sound.

(OK, to be fair, σ before μ sounds like a ζ...)

[Stergi3]

Ζ, ζ is always [z]. About Σ,σ,ς: It depends on the following letter, even it is in the next word. Before voiced consonant it is [z]. Voiced consonants in Modern Greek are: β, δ, ζ, γ, (ν), λ,ρ,μ, ν and μπ, ντ, τζ, γκ. Εxception is λ. So i.e. σβούρα=[z]voura, σμπάρος=[z]baros. See the classical book of Greek Grammar by Manolis Triantafyllidis, page 8: https://www.alfavita.gr/sites/default/files/attachments/grammatiki.triantafyllidi-sc2013.pdf in the document. In all other cases Σ,σ,ς is [s]

[kirakrakra]

Oh, it is the grammatics of the great Greek linguist Manolis Triandafyllidis. Thank you very much

[Windrammer]

Not really helpful. After some searching I found something.

[Stergi3]

Sorry, if it didn't help much. The above Greek Grammar is a reference book though. Just it written in Greek. It is a complete guide, even the native Greeks don't know the grammatical phenomena included in this book. Unfortunately in Greek.