When I was in Missouri in about 1971 for a Son family reunion, the story that was told to me at that time was this:
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That our name was originally Sohn and the preacher took the h out, guess for perhaps religious reasons, not sure.
They said the name Sohn was German and that is where the original Sohns had come from.
Not sure if this is any help to anyone, but I put the name Sohn in the FamilySearch engine and came up with nothing for Spain, but found several Michael Sohn's from Germany.
Just a thought.