The appropriate pronunciation of the plural ending depends on how the word ends, phonemically. The rules are as follows:
* For words ending with a vowel sound or a voiced consonant sound, the correct pronunciation of the plural ending is /z/
* Words ending with a voiceless consonant sound the appropriate pronunciation is /s/
However, some phonemes require the /IZ/ pronunciation. Unfortunately I cannot type the phonetics here, though. Words ending with phonemes /s/ and /z/ are included in this group.
NB! Irregular plurals are sometimes pronunced differently from what is described below.