In some languages, a voiced consonant may make a previous voiceless consonant voiced. This is called voicing assimilation. However, this process does not happen in English.
Voiceless consonant /s/ Voiced consonant /b/
Nice boy Nize boy
Voiced consonant /z/ It can also happen with more than one consonant:
Let’s go Ledz go
Difficulty making the voiceless consonant? Make the previous vowel shorter.