Thank you for not forgetting me
I found the problem is not only perceiving it correctly, the problem is also drawing the border between two sounds at the correct place, if that makes sense.
If you have two similar sounds, the first (1) at the left, the second (2) at the right, there's a linear continuum between them.
1 ........ | ....................... 2
Where you draw the border (|) might be different in two languages.
In my native language it's usually okay to substitute /z/ with /s/. In fact, the German letter z is almost always pronounced /ts/, so "Zoo" is pronounced /tso/ in German, while the German letter s is sometimes /s/, sometimes /z/, depending on the dialect. This is why, to my ears, both the English /s/ and /z/ sounded like /s/ to me (with the exception of a really strong /z/). I, my girlfriend, and my parents pronounce most German words that have the letter s in them with an /s/, while my German dictionary says for the same words that it's a /z/, and if I pronounce it with an /z/ it still sounds German. It usually doesn't matter, my ears became more or less indifferent to the difference.
But when I finally payed attention and notice how an English native speaker pronounces the /s/ phone, it struck me that it is indeed different from even an only slightly voiced English /z/ phone.
So, I
can now hear the /z/ in the first audio example of the woman I attached in an earlier post. Which makes me happy. :-D
I perceived it all the time, I just "drew" the border between /s/ and /z/ at a different place than you guys do. I just need more practice to hear the difference effortlessly.
The /ʧ/ vs /ʤ/ at the end of a word still causes problems for me, however. I can hear the difference at the beginning of a word, so I can distinguish chew vs Jew, but at the end of a word where /ʤ/ may be devoiced in normal speech, I find it difficult to hear.
To illustrate my problem I again have attached two audio samples. One has an /ʧ/, the other a devoiced /ʤ/. Please try to listen to each sample only once before you make up your mind on which is which. I'd be interested if you hear a difference at the first attempt.
I can make out which is which, but I doubt I could do it in normal conversations.