In both those cases, the letter 's' is pronounced as a /z/, as if it were 'Adamz' and 'Lilyz'. A final 's' is only pronounced as an /s/ after the unvoiced consonants f, k, p and t, and also the sounds represented by 'sh' and 'ch', and the 'th' sound at the end of 'Beth', for example. Mick's = /mɪks/ The 's' is pronounced as an /s/ because the /k/ is unvoiced. Meg's = /megz/ The 's' is pronounced as a /z/ because the /g/ is voiced. Note that vowels count as voiced sounds. For example: Kit's = /kɪts/ The 's' is pronounced as an /s/ because the /t/ is unvoiced. Kitty's = /kɪtɪz/ The 's' is pronounced as a /z/ because there's a vowel sound before it. These rules also apply to all 's' sounds at the end of words, including those used for plurals and third-person present-tense verbs. By the way, it's really worth learning to train your ear (and your mouth) to these phonological patterns. I have met many people who are almost bilingual but who consistently mispronounce the final /z/ sound as an /s/. This is a typical error of native German and Spanish speakers, for example.

Su.Ki. gave an excellent answer. To extend it a bit. '-s' is pronounced [s] after non-sibilant unvoiced consonants for plurals, verbs in the third person singular present, contractions, and the genitive. two cats [kæts] He puts [pʊts] on his hat It's [ɪts] easy Pat's [pæts] book '-s' is pronounced [z] after non-sibilant voiced consonants and all vowels for plurals, verbs in the third person singular present, contractions, and the genitive. two pins [pɪnz] He pins [pɪnz] a ribbon to his hat He's [hiz] late for work. Sue's [suz] book '-s' is pronounced [ɪz] for sibilants. Often the spelling changes 's' to 'es'. two horses [hɔrsɪz] The bee buzzes [bʌzɪz]. The church's [tʃɜrtʃɪz] pastor.

It's not quite as severe as a 'z', but 's' is sounded in more than one way in English. Here, Google Translate can say it out loud for you. Click the speaker icon on the English side. https://translate.google.fr/?hl=en&tab=rT1#view=home&op=translate&sl=en&tl=zh-TW&text=Adam's%20apple

@Catcher, 多谢你的建议：）

In my opinion they sound more like s, 其實像這種物主代詞 表示它前後從屬關係的s（adam's /lily's/his/）在整個詞組裏只是很輕的一個輔音，説出來的時候一般就是蜻蜓點水地過去，不需要太糾結這一點哦。但是個人意見是當然不要太偏向z，不然這個發音會聽起來太重，不自然。;)