The /s/ and /z/ sounds are made by holding your teeth partially open and lifting the front of your tongue (right behind the tip) and hold it close to the ridge between your teeth and the roof of your mouth. The result is a hissing sound. The difference between the /s/ and the /z/ phonemes is that /z/ is voiced, and /s/ isn’t.
The /s/ or /z/ sounds can occur anywhere in a word. Examples of /s/ at the beginning of a word are “sit,” “still,” “sun.;” examples in the middle of a word are “essence,” “best,” and “mist;” examples at the end of a word are, “kiss,” and “boss. Examples of the /z/ sound at the beginning of a word are “zest,” “zap,” and “zoo;” in the middle of a word are “fuzzy,” “raised,” “teased,” and “frozen;” at the end of a word “bees,” “flees,” and “trees.”
It is useful to notice that though the words are spelled with an ‘s’ at the end, the sound would be spelled “z” if English spelling represented the sounds.
In the section on morphemes, there is a subsection of English plurals and the third person singular verb. They are spelled by adding “s” or “es” to words, but when they are spoken, they are pronounced /s/, /z/, or /əz/. This predictable with simple rules, the sound is /s/ when the word ending is voiceless and not sibilant, in other words, /k/, /p/, and /t/ as in / “leaks” /liks/, “peeps” /pips/ and “cats” /kæts/, After sibilants, /s/, /ʃ/, /z/ and the affricates /t͡ʃ/ and /d͡ʒ/ the sound is /əz/ as in “kisses” /kɪsəz/ , “dishes” /dɪʃəz/, “buzzes” /bʌzəz/, “catches” /kæt͡ʃəz/, and “badges” /bæd͡ʒəz/. After voiced sounds that are not sibilants the ending is pronounced /z/. This is a long list and includes the 13 vowels and the consonants /b/, /d/, /g/, /l/, /m/, /n/, /r/, /v/ as in “cubs” /kʌbz/, “dads” /dædz/, “dogs” /dɑgz/, “bills” /bɪlz/, “beams” /bimz/, “beans” /binz/, “beers” /bɪrz/, “moves” /mʉvz/.