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When do you pronounce ‹s› as /z/ in the middle of words? Is there any rule? I also saw there are some differences in articulating medial s between American and British accents. I already know the rule for pronouncing plural s and es.

Is there any case where ‹c› is pronounced /z/?

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I couldn't find good references by Googling, and I don't know anything about British english. As I think it through, it is quite complicated! Sorry -- we should really get around to some spelling reform. I hope others can help edit this list if they think of exceptions.

In American English, typically

  • If there are any prefixes or suffixes causing an s to be in the middle of a word (either because the "s" is part of the prefix or because it is part of the root"), the "s" is always unvoiced /s/, e.g. subsist, substandard, mismatch, mistake, etc.

  • An s that is written next to an unvoiced consonant is always unvoiced /s/, e.g. lisp, rasp, history, etc.

  • When the unvoiced consonant of the above rule is [t], then the /t/ is silent if the next syllable is syllabic /n/ or /l/: listen, whistle. (Otherwise it is pronounced. See the comments for a more detailed description of this rule.)

  • An s before m is always voiced /z/: chasm, prism, plasma. However, the top rule takes precedence, so the s in mismatch is always voiceless /s/. I cannot think of another letter besides [m] where s occurs before or after a voiced consonant word-medially (except by prefixing as in example one), but it would be voiced in such a situation.

  • An s that is written doubled between vowels is also unvoiced (I guess this is a special case of the above rule): massive, missive, missile, etc. However, if the s would occur in the phonetic stream /sj/ then it assimilates to /ʃ/, e.g. in mission.

  • An s that is written as one single letter between vowels is usually /z/, e.g. laser, risible. In the same environment as mentioned above /zj/ will assimilate to /ʒ/ e.g. in vision.

  • Terrible exception to the above: in dessert, the s is voiced to /z/. Many native English speakers misspell dessert for this reason. Note also that the difference between desert and dessert is not voicing, but which syllable gets the accent (it is the first in desert and the second in dessert).

  • Possess and its derivatives are another exception; the middle "ss" is voiced to /z/. The terminating "ss" is not.

  • Other miscellaneous exceptions: The -ss- in the American state name Missouri is also exceptionally pronounced /z/. In raspberry, the p is silent and the [s] assimilates to the /b/, so is voiced to /z/.

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as to your other question, orthographic [c] always represents an unvoiced consonant (exception: czar, I'm sure there are others if we allow relatively recent borrowings from Slavic languages). Whether that consonant is /s/, /k/, or /ʃ/ will depend on the next letter: if it is written [a, o, u] the c is /k/; if it is written [e, i, y] the c is /s/, except that as usual /sj/ assimilates to /ʃ/; so, cell, call, ocean. – hunter Jul 29 '14 at 10:24
+1 for thoroughness. The rule for "st", though, feels more like an exception than a rule, e.g.: "paste", "trust", "abstention", "assist", "assisted", "blustering"... I found "christendom", "chastened", "fasten" are like "listen". Another exception is Leicester... I think this is a tough one to describe! – Nico Jul 29 '14 at 10:43
yes, [st] prononuced /s/ is exactly when the next syllable is syllabic /n/ or syllabic /l/; this is what was already written above and is consistent with all your data (except Leicester -- place names are hopeless! I was going to add a bunch more place names to the "exceptions" rule and then gave up.) – hunter Jul 29 '14 at 10:45
I think I don't understand what you mean by syllabic /n/ or /l/. Does the condition syllabic /n/ exclude "abstention"? – Nico Jul 29 '14 at 10:51
yes. the nucleus of the syllable in abstention is /$\epsilon$/ and not the n. en.wikipedia.org/wiki/Syllabic_consonant – hunter Jul 29 '14 at 11:28

Some additional guidelines beyond Hunter's:

  • An s in the sequence -ns- is almost always unvoiced /s/, e.g. insist, tense, tinsel. This doesn't apply to words that end in -ns, like pens or lens (these have /z/). Exception: cleanse, which has /z/.

  • An s in the sequence -rs- is almost always unvoiced /s/, e.g. persist, verse. This doesn't apply to words that end in -rs, like stars or yours (these have /z/).

  • An s in the sequence -ls- is almost always unvoiced /s/, e.g. else, pulse. This doesn't apply to words that end in -ls, like eels or steals (these have /z/).

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