s发/s/的读音规则
一、s在词首时;除了sugar;sure以及sh组合发/ /以外;其余一般发/s/.. 例：
surface;serve;seven;six;some;sign
比较：design/di'zain/
一般前缀;合成词不影响其读音..s仍读成/s/..例：
unsafe;unsatisfactory;roadside;teaspoon;snowstorm
二、两个s在词尾时读作/s/.. 例：
grass;glass;address;press;success;pass;miss;stress;across;swiss; progress;process;kiss
三、词尾s在u后读作/s/.. 例：
bus;us;minus;abacus;plus;status;virus
四、在sis组合的弱读中;两个s都读作/s/.. 例：
emphasis;analysis;thesis;crisis
emphasise; emphasize; analyse/ analyze; criticise/criticize criticism n.
五、s在字母c前常读作/s/.. 例：
muscle;discipline;science
六、s在某些前后缀中读作/s/..
1．在前缀mis-;dis-中;s读作/s/.. 例：
mismanage;misjudge; misbelieve;disorder;disobey
2．在后缀－sive;sity;－self;-some;-sy中;s读作/s/.. 例：
expensive;intensive;decisive; yourself;handsome;troublesome;tiresome;quarrelsome
七、s在清辅音前后常读作/s/..
1．s在清辅音前常读作/s/.. 例：
honest;newspaper;task;satisfy;grasp grab; grip
2．s在清辅音后常读作/s/.. 例：
sportsman;works;stops;roofs．
八、词尾se在字母r;l;n后读作/s/.. 例：
horse;nurse;worse;course; universe;pulse;else;tense;sense
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清音/s/; e.g. subsist; substandard; mismatch; mistake; etc.
An s that is written next to an unvoiced consonant 清辅音 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有优先权; so the s in mismatch is always voiceless /s/.
An s that is written doubled between vowels is also unvoiced: massive; missive; missile; etc. However; if the s would occur in the phonetic stream /s+j/ then it assimilates to / /; e.g. in mission.
An s that is written as one single letter between vowels is usually /z/; e.g. laser; risible; criticise/ criticize; desert; design; reason; busy; result; reserve; closer the comparative form of the adjective "close"; has an /s/ sound. In the same environment as mentioned above /z+j/ will assimilate to / / e.g. in vision.   
piano怎么读
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/.
补充：Based on the immediately surrounding letters:
Word-internal -ns-; e.g. in insist; tense; tinsel; is almost always pronounced /ns/ with unvoiced /s/. This doesn't apply to words that end in -ns; like pens or lens these have /nz/. Exceptions: cleanse and pansy; which have /nz/. For some speakers; certain but not necessarily all words starting with "trans" such as transit and transition have /nz/.
Word-internal -ls-; e.g. in else; pulse; is almost always pronounced /ls/ with unvoiced /s/. This doesn't apply to words that end in -ls; like eels or steals these have /lz/. Exception: palsy; which has /lz/.

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