Just a question out of curiosity. Before typing became commonplace, many writing communications must have been transcribed from dictation. My hypothesis is that certain syntax and grammar must change from dictated to (personally) written communication. Is there any research in this subject?
The problem you are thinking about may have to deal with linguistic recast which involves transformation of non compact speech(like spoken discourse) to or from compact metaphorical discourse(like a formal Essay). Related work on functional recast is by systemic functional linguistic scholars Mohan & Beckett they present two kinds of recasts: corrective recast which grammatically corrects the input speech and functional recast which recasts by paraphrasing the discourse rather than correcting syntactic mistakes. Ping presents example study of functional recast from textbook discourse to spoken discourse.
Certainly spoken syntax differs from written syntax; but dictation is a linguistic skill which can be learned like any other. Some considerations to keep in mind
In the heyday of dictation, most dictated communications were addressed to very routine matters for which the originator had developed a lexicon of stock phrases and sentences. "In response to yours of the 4th inst. I have the very great pleasure of informing you that ... "
By the same token, secretaries were skilled at ignoring the false starts and local solecisms endemic in speech, and took upon themselves - were indeed often relied upon - to rewrite as they transcribed. "Take a letter, Miss Samuels. My dear Harper, I was very happy to hear that, blah, blah, blah - you know what to say. Stick in something about his speech at the Chamber ... "
Finally, dictation was often only a first draft: an important letter or speech was dictated, typed, and then corrected, perhaps many times over.
Moreover, I would be surprised to find that dictation was more common before typewriting. My impression is that before the end of the 19th century bosses composed in longhand and handed off their drafts to clerks for fair-copying (twice: once for the recipient and again in the firm's letterbook). It was not until after the advent of the new technologies typing, shorthand and carbon paper - and of that new and exceptionally cheap IT drudge the female secretary - that bosses found dictation a more efficient use of their time.