why is the letter s in "she cries" pronounce like "z" and in "she asks/tells" like "s"?

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The rule for adding -s to words:

1) If the word ends in /s/, /z/, /ʃ/, /ʒ/, /tʃ/, or /dʒ/:
- add -/əz/
- e.g. "kisses," "pleases," "leashes," "catches," "badges"
2) If the word ends in /p/, /t/, /k/, /θ/, or /f/:
- add -/s/
- e.g. "leaps," "eats," "asks," "laughs"
3) In all other cases:
- add -/z/
- e.g. "cries," "tells," "finds," "leaves," "sings"

These rules make pronunciation easier. Even though it is spelled with an s, the s at the end of these words is really a z. In situations where it would be hard to pronounce the z as a z, it changes its pronunciation. In rule (1), for example, z would follow a sound that is too similar to it, so a vowel is inserted. The sounds in rule (2) are "voiceless" sounds, while z is "voiced." Since its hard to say a voiceless sound and a voiced sound next to each other, z changes to s, which is voiceless.

* the letters between / / are international phonetic alphabet (IPA). If you want to learn more about IPA (assuming you don't know it already), you can google it or check out this link: https://en.m.wikipedia.org/wiki/Help:IPA/English)