And the LORD opened the mouth of the donkey, and she said to Balaam, "What have I done to you, that you have struck me these three times? Numbers 22:28
“Just because we cannot see the spiritual does not mean that we should be unaware of its presence. After opening the donkey's mouth so that it could speak, God opened Balaam's eyes so that he could see the angel too. The implication is that ordinarily we are no more capable of seeing spiritual beings than animals are capable of uttering human speech.
The story of Balaam's talking donkey makes a popular children's story. But it is not a fairy tale. The [Old Testament] relates the conversation in a matter-of-fact manner, simply explaining that "the LORD opened the mouth of the donkey" (Numbers 22:28). The implication seems to be that all animals would be capable of speech if only God opened their mouths. Remember the talking serpent in Eden?
The miracle of the talking donkey is not just for amusement. It is an illustration to prepare us for the [verses] that are about to follow. In Numbers 23-24 Balaam inadvertently speaks a series of blessings and prophecies over the people of Israel. Balaam's prophecies are directly from God. How could God speak through the wicked sorcerer Balaam?
The lesson of the talking donkey illustrates that God is not limited to using only righteous men to deliver His messages. If He can use the mouth of a donkey to get a message across, He can also use the mouth of a wicked man like Balaam. In a sense, Balaam plays the role of God's talking donkey.”
First Fruits of Zion Parasha: Balak