This morning on Weekend Edition Sunday, NPR reporter Mike Pesca, talking about strikeouts and curveballs, introduced the Higgs boson as a metaphor and ran it into the ground. All through this, he gave boson the accent pattern primary accent + unaccented, rather than the standard pattern primary accent + secondary accent. Along with the accent difference went the voicing of the medial fricative (spelled S); he had a medial /z/ every time, instead of the standard /s/:
standard /bósàn/, Pesca /bózǝn/
Presumably, for Pesca the word boson had become so familiar that the final syllable was deaccented, as in outsider-pronunciations of Oregon with /àn/ vs. Oregonian pronunciations with /ǝn/. And then the medial fricative got voiced, as in reason, raisin, besom, and some other words spelled with medial S. But there is a strong competing pattern, with /s/ (rather than /z/) for words that have unaccented final syllables and are spelled with medial S: mason, bosun (a minimal contrast to Pesca’s boson), basin, bison, and so on. (I’ve chosen words with tense vowels in the first syllable, to avoid the issue of spellings with medial S vs. SS.)
There are histories for each of these words, but it’s clear that synchronically the voicing isn’t determined automatically by phonological context. So I’m not sure what led to Pesca’s choice of /z/.