I have been puzzled by the question for a very long time.
Let's assume that the following words are all people's names.
Jack, Jat, Jass, Jaz,
1. This is Jack's book.
2. This is Jat's book.
3. This is Jass's book.
4. This is Jaz's book.
5. This is my teacher's book.
6. These are our teachers' books.
How to pronunce those red parts?
Now I read them as /ks/, /ts/, /siz/, /ziz/, /z/, /z/.
Am I right?
I doubt these because I often hear "'s" is pronunced as "/s/", even in the 5th and the 6th sentence. ( I think in the last two sentences, it should be pronunced as /z/)
So, are there any rules?