If a word ends in the letter 's' (e.g. plural noun or verb in the third person), there are three ways to pronounce this 's' - /S/, /Z/ and /IZ/.
1. Final 's'
In type one words, the final 's' should be pronounced with air pushed out between the top of your tongue and your top teeth, without using the vocal chords (the sound made by a snake or gas escaping from a pipe).
Type one sounds are used if the word ends in the following sounds:
These sounds are all unvoiced, which means that your vocal chords must be silent when you make the sound.
Type two sound /Z/
This sound is formed in the same way as type 1, but you must make a sound with your vocal chords (the sound made by a bee, or high-voltage electricity)
Type two sounds come after the following sounds:
any vowel sound
any vowel sound
plays, employees, flees, goes, news
These sounds are all voiced, which means that your vocal chords must make a sound when you pronounce them.
This sound is the same as the verb 'to be' in its third person form (is). Type three sounds are the only ones which add an additional syllable to the word, for example 'miss' /MIS/ is one syllable, but 'misses' /MIS IZ/ is two syllables. The final /IZ/ syllable isn't usually stressed.
Words ending in 'y' usually have a type 3 sound, but there is no extra syllable e.g. 'copy' /KO PEE/ (2 syllables), 'copies' /KO PIZ/ (also 2 syllables)
English pronunciation rules - sounds by Viv Quarry (www.vivquarry.com)
Please check these links for further clarifications:
Word Final /-s/ - Authentic American Pronunciation
CCC Pronunciation Tips