Numbers 22:28 - Albert Barnes' Notes on the Bible

And the Lord opened the mouth of the ass - The account was perhaps given by Balaam to the Israelites after his capture in the war against Midian. Compare Numbers 31:8. That which is here recorded was apparently perceived by him alone among human witnesses. God may have brought it about that sounds uttered by the creature after its kind became to the prophet’s intelligence as though it addressed him in rational speech. Indeed to an augur, priding himself on his skill in interpreting the cries and movements of animals, no more startling warning could be given than one so real as this, yet conveyed through the medium of his own art.