There’s a very confusing aspect of the ‘S’ sound in American English. ‘S’ has more of a light ‘Z’ sound in some common words when the ‘S’ follows a vowel.
The phonetic rule is like this:
If ‘S’ follows a vowel or a voiced consonant, it sounds like ‘Z’.
However, this rule is not constant, there are some exceptions. For example, “is” has the ‘Z’ sound, but “this” does not. Remember, just like the flap T sound rules, most Americans are unaware of this distinction. When asked what sound is made at the end of “is”, they would say ‘S’, and then speak it with a ‘Z’ the next thing they say.
Watch this video from Rachel. She will teach you How to Pronounce S ‘z’.
Now practice your pronunciation with the following words:
rise – raise
pose – pause
these – those
his – has
flies – flows
says – days
easy – busy
confuse – confess