[Facts] Re: Joseph - how do you pronounce it? (and where are you from?)
"The normal English pronunciation of /s/ between vowels and voiced consonants is /z/" is an overgeneralization.
There is a rule that /s/ is voiced to /z/, without exception, when it marks the plural or 3rd-person singular of a verb and follows a vowel or a voiced sound (e.g. dogs, combs, runs, goes). But this rule can't be extended to other applications. Sometimes /s/ is voiced to /z/ between voiced sounds in other words, and sometimes it is not (e.g. basis, crisis, precise, usage, also, et al.) So your statement is not helpful in determining whether "Joseph" is or should be pronounced with /s/ or /z/.