In American English, the sound of "s" depends on which sound comes before it.
1. If the noun ends in an unvoiced consonant sound: /f/, /k/, /p/, /t/, /th/-(thin), pronounce "s" as /s/.
2. When it ends in a voiced consonant sound, /b/, /d/, /g/, /l/, /m/, /n/, /ng/, /r/ or with a vowel...