Pronunciation of Σίγμα and Ζήτα.
How does one produce these sounds? They're not quite like sibilants found in most Euro languages.
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...)
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]