John Gill Commentary Jeremiah 36:13

"Then Micaiah declared unto them all the words that he had heard, when Baruch read the book in the ears of the people." — Jeremiah 36:13 (ASV)

Then Micaiah declared all the words that he has heard The sum and substance of them; for it cannot be thought that he should retain in his memory every word that he has heard; though, as it is very probable he was much struck and affected with what he has heard, he might remember and declare a great deal of it.

when Baruch read the book in the ears of the people and this he also declared, no doubt, that what he has heard, and then related, were read by Baruch out of a book; as is clear from the princes sending for Baruch, and ordering him to bring the roll along with him, as in (Jeremiah 36:14) .