Voicing s as /z/ is widely variational. For example, I voice the s in "us", and "Muslim", while others don't. While I don't voice the s in Australia (/ɒ.ˈstreɪ̯.li.jə/ ~ /ə.ˈstreɪ̯.li.jə/), I do in Oslo (pronouncing it /ˈɒz.loʊ̯/).