Jamieson, Fausset and Brown on Numbers 22:28

28. the Lord opened the mouth of the ass--to utter, like a parrot, articulate sounds, without understanding them. That this was a visionary scene is a notion which seems inadmissible, because of the improbability of a vision being described as an actual occurrence in the middle of a plain history. Besides, the opening of the ass's mouth must have been an external act, and that, with the manifest tenor of Peter's language, strongly favors the literal view [2Pe 2:15, 16]. The absence of any surprise at such a phenomenon on the part of Balaam may be accounted for by his mind being wholly engrossed with the prospect of gain, which produced "the madness of the prophet" [2Pe 2:16]. "It was a miracle, wrought to humble his proud heart, which had to be first subjected in the school of an ass before he was brought to attend to the voice of God speaking by the angel" [Calvin].