From Hull AWE
The written inflection '-s' (or '-es') represents different sounds:
- the voiced '-z' (IPA: /z/) after other voiced consonants ('-b-', as in robs; '-d-', as in pads; and '-g-', as in dogs; etc) and after vowels, as in radios and such vowel sounds as plays, toys and cries;
- the unvoiced '-s' (IPA: /s/) after unvoiced consonants ('-p-', such as stops; '-t-', as in flats; '-k-', as in speaks; etc);
- the syllable '-iz' (/ɪz/) after sibilants, as in the disyllables kisses (/'kɪs ɪz/) and boozes (/'buːz ɪz/).
You may also want to see -ed (phonetic), for a similar pattern.