Albert Barnes Commentary Numbers 22:28

"And Jehovah opened the mouth of the ass, and she said unto Balaam, What have I done unto thee, that thou hast smitten me these three times?" — Numbers 22:28 (ASV)

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 recorded here was apparently perceived by him alone among human witnesses. God may have brought it about that sounds uttered by the creature of its kind became to the prophet’s understanding as if 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 as real as this, yet conveyed through the medium of his own art.