Consider these English words, all borrowed from Latin or from Greek via Latin. How is the final consonant pronounced?
-as: paterfamilias, Judas, atlas, Maecenas;
-es: tabes, faeces, Hades, bona fides, scabies, rabies, species, series, obsequies, isosceles, anopheles, herpes, lares et penates, Euripides, Sophocles, diabetes, Dives;
-is: cannabis, ibis, aegis, aphis, corydalis, chrysalis, annus mirabilis, syphilis, torticollis, acropolis, metropolis, epidermis, lenis, penis, Adonis, ex-libris, hubris, Berberis, iris, clitoris, basis, emphasis, psoriasis, oasis, stasis, exegesis, thesis, genesis, diaeresis, enuresis, crisis, thrombosis, silicosis, psychosis, osmosis, diagnosis, sclerosis, neurosis, halitosis, ellipsis, synopsis, catharsis, analysis, paralysis, appendicitis, bronchitis, mantis, glottis, axis, lexis;
-os: chaos, logos, pathos, ethos, thermos, cosmos, tripos, rhinoceros;
-us: syllabus, omnibus, incubus, abacus, diplodocus, focus, locus, discus, mucus, exodus, nucleus, Theseus, Zeus, sarcophagus, asparagus, fungus, typhus, polyanthus, ceanothus, radius, regius, genius, angelus, nautilus, phallus, gladiolus, calculus, tumulus, stylus, hippopotamus, isthmus, animus, humus, anus, tetanus, Venus, tetanus, genus, terminus, alumnus, campus, opus, Lazarus, virus, plesiosaurus, papyrus, Pegasus, Croesus, census, versus, narcissus, apparatus, status, f(o)etus, tinnitus, coitus, emeritus, nexus.
In the case of words ending in -as, -is, -os or -us, the final consonant is a voiceless /s/. But with those ending in -es it is a voiced /z/.
I have no idea why there is this difference. But it corresponds to how we pronounced Latin when I was at school. Dominus was ˈdɒmɪnəs, librīs was ˈlɪbriːs, but spēs was speɪz.
In classical times Latin s represented a voiceless sound in all positions (Allen, Vox Latina, p. 35): so there’s no justification there for our English voiced /z/ in series and diabetes. I wonder where it came from.