Does the way the TTS in this course pronounces ζ and σ sound natural to native speakers? These are certainly not the clear /s/ and /z/ sounds. All the recordings I've heard today except this TTS sound "normal".
Wikipedia says that:
/s/ and /z/ are somewhat retracted ([s̠, z̠]); they are produced in between English alveolars /s, z/ and postalveolars /ʃ, ʒ/. /s/ is variably fronted or further retracted depending on environment, and, in some cases, it may be better described as an advanced postalveolar ([ʃ̟]).
So it kind of makes sense. But still, does this pronunciation sound natural to native speakers? (I'm talking about ζ in this particular sentence. σ is pronounced as quite clear here, unlike in other sentences)
But when I compare it with other recordings like some here I can hear a huge difference. To me, it sounds extremely similar to Polish /ʐ/ (usually "softer" than in Russian). Maybe this is the reason, because there are no other similar sounds in Greek and your z has "more space" while an ear of a Slavic language speaker like me is more senstitive to these changes. I've also had this problem with Dutch and it's been really bothering me for a long time. I guess I have to take this as an explanation :D Thank you for your help!