Learn English – What are the Correct pronunciations for “Wednesday” and “pizza”

The two are fairly interchangeable, in my opinion as a native [British] speaker. When emphasizing, we tend to use use [eɪ] more often than when not emphasizing. [ə] is more common in general, but really there's not much in it. [ə] can be dragged out more than [eɪ] when hesitating.

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/.