When an -S is added at the end of a word, either for the plural or the third person of the singular with the present it does not always sound the same. The pronunciation of -S depends on the final consonant sound. if the ending vibrates it should be pronounced /z/. if it does not it should be /s/.
You can check if a sound vibrates or not by putting your hand on your throat. Careful, vowels always vibrate! Try to pronounce the consonant only. You can check which consonant vibrates or not on the phonetic alphabet.
The last case is when the final consonant sound is a sibilant which is characterized by a hissing sound. Sibilants are the following sounds: /s/ in kiss, /z/ in buzz, /ʃ/ in wash, /tʃ/ in beach and /dʒ/ in judge.
