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The normal English pronunciation of s between vowels is z. The same thing happens in words such as trousers, houses and housing (but not house), bowser, wowser, Aussie, etc.. There are some exceptions (e.g. buses) where the vowel is effectively unvoiced, and final s is also voiced to z following another voiced s and some some vowels (and in words where the s was originally not final but have been shortened so it is, such as rouse, carouse). As the value of s is positional, there is no z in native English words except for more recent onomatopoeic words such as buzz, and "reformed" spellings such as booze (from bouse, from the Middle English verb bousen, in which the s is voiced). Essentially if there is a voiced sound (most vowels and some consonants) before and after the s, the s is also voiced as IPA /z/, but if the following letter is not voiced, then it pronounced as IPA /s/.