The letter s represents two sounds in English, the unvoiced sound /s/ heard in sister, and the voiced sound /z/ heard in rose.
Note: The “voiced” s sound buzzes, like the sound of /z/. The unvoiced s sound is “soft,” like the hiss of a snake.
When the spelling combination -ise occurs in verbs and nouns, the s is usually voiced, but not always.
Verbs in which the s in -ise is voiced:
compromise (BUT: promise with /s/)
While on the subject of -ise endings in verbs, a comment is probably in order on the widespread misconception that the verb ending -ize as in synthesize is an American corruption of lovely English verbs that “should” end in -ise. The OED recommends the -ize ending for verbs that derive from Greek, and, as demonstrated by the list above, Americans have not altogether abandoned the -ise ending. The usage differs only with verbs in -yze, like analyze and paralyze. OED prefers analyse and paralyse for etymological reasons.
In its entry on the suffix -ize, the OED states the rationale for preferring the -ize spelling:
the suffix itself, whatever the element to which it is added, is in its origin the Greek -izein, Latin -izāre; and, as the pronunciation is also with z, there is no reason why in English the special French spelling should be followed, in opposition to that which is at once etymological and phonetic.
For a fascinating comparison of views on the subject of -ize vs. -ise, see Mike Horne’s “The -ize have it!”
Nouns in which the s in -ise is voiced:
Nouns in which the s is unvoiced:
treatise (a secondary pronunciation with /z/ is given in the OED)
Some of you may be surprised to see expertise listed with the nouns in which the s has the /z/ sound. The pronunciation of expertise with the unvoiced s is rampant on American television, but both the OED and Merriam-Webster Unabridged agree that the pronunciation is [EK-spur-TEEZ]. Some other dictionaries now list the unvoiced pronunciation as an acceptable alternative, but Charles Elster is having none of it:
”Careful speakers should make an extra effort to hold the line on this one [EK-spur-TEEZ].” –The Big Book of BEASTLY Mispronunciations, page 189.