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Icelandic patronymic pronunciation

I find the pronunciation of Icelandic highly regular and predictable on the whole, but male patronymics continue to puzzle me.  The suffix "-son" is consistently pronounced with an initial /ʃ/ rather than /s/, for example [here](https://forvo.com/word/haf%C3%BE%C3%B3r_bj%C3%B6rnsson/).  Also, and possibly related, I'm unable to tell why the "son of Jón" would be written as "Jónsson" rather just "Jónson".

What rules govern the spelling and pronunciation of the initial sound in the "-son" suffix?

Parallels in other North Germanic languages might be interesting, too, as long as they throw some light on modern Icelandic.