This question is directly related to my former question on Aramaic:
How to learn the vocabulary of an ancient language like Aramaic, which is based on scripture only?
I'd like to know how and on what basis people could learn or be sure that the pronunciation is correct?
Was the pronunciation somehow "preserved"?
Or was the pronunciation somehow taken from other languages which are still in use?
In the first chapter of his book Philology ((c) 2014 Princeton University Press), James Turner describes the notation systems created by Zenodotus of Ephesus, the first "librarian" (ca. 270 BCE) of Ptolomy II's massive library at Alexandria, Egypt. Zenodotus developed his system to help edit the massive number of papyri originals and copies of works by ancient and contemporary authors that were being deposited in the library's collections.
Two of Zenodotus' successors as librarians, Aristophanes of Byzantium (ca. 257-180 BCE) and Aristarchus of Samothrace (ca. 216-144 BCE) refined Zenodotus' editing systems and procedures. Those editing systems were found to be necessary to sort out and classify the plethora of good and bad 'translations' of ancient Greek (and Aramaic) texts and gospels included in the Hebrew and Christian bibles.
Zenodotus, Aristophanes, and Aristarchus are credited with devising coded symbols to evaluate texts. For example, a horizontal pen stroke in the left margin, an obelus (a spit or skewer) marked doubtful (as to veracity or origin) of the text, the right arrow > (diple) indicated 'something worthy' in the text, and the asterikos * marked incorrect copy -- of which there was much, particularly in copies sold by less than honest book sellers as being copied 'from the original'. According to Turner, agents for the library were also sent out to "buy or hijack books from ships docked at Alexandria, or acquired them by "bald-faced deceit", many of which were undoubtedly corrupt copies of originals.
Zenodotus is also credited (by Turner) as being the original inventor of listing defined words in alphabetical order in glossaries appended to books.
Aristophanes of Byzantium himself is credited with using accent marks (acute, grave, and circumflex) to aid scholars in the pronunciation of words. He also used commas, colons, and periods to help the reader know how long to pause in a text while reading it.
Turner also notes that Aristarchus' student Dionysius Thrax (ca. 170-90 BCE) may have been the first scholar to write a book on grammar.
It is clear from Turner's work that scholars with cross-cultural and cross-disciplinary knowledge of not only languages, but also knowledge of the cultural norms of the speakers of languages under study at the library of Alexandria helped enormously to insure the correctness of texts being translated and transcribed and who, at the same time, helped to preserve "for posterity" the languages which they were transcribing.
