Welcome, @Silke! Yes, this is what usually happens to the sound of the letter S:
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When followed by a vowel in fast speech, it sounds like a Z (as in zero) and there’s a kind of “liaison” with the following word:
Desculpe, mas eu não entendo => Desculpe, ma[z]eu não entendo (I’m sorry, but I don’t understand) -
When followed by a vowel in slow speech, the more each word is pronounced independently, the less apparent is the Z sound and “liaison”, so the S might also sound like SH (as in shoe) here.
Tens… um… livro? => Ten[sh]… um… livro? (Do you have a book?) -
When followed by a consonant, it also sounds like a SH (as in shoe).
Falas português? => Fala[sh] português?