During the scene in parshat Balak, it says that "God opened the mule's mouth" and it asked Bil'am why he hit him. Bil'am answers the donkey. It seems that he is not surprised at all that the donkey is even asking him this question or talking to him! Did Bilam understand this was some miracle and that this could happen, somehow? What's going on that Bil'am doesn't even seem surprised at what's happening?
In otzros hatorah page 263 on "vayiftach hashem es pi haton" rabenu bechayeh is quoted asking that Bilam should have been amazed but he was not. He should have marveled at this wonder. However because of his cruelty and evil nature and desire to go curse the Jews, he answered the donkey's question. It continues that from here you can understand why people say that if we would see great miracles, we would return to hashem. However we would act in the same way. Rav Yisroel Salant would ask why people who work in burial of people don't live with the concept of remember the day of death. He answered why don't we wonder the same about the horses that transport the dead bodies? That was Bilam who acted like those horses and unfortunately many times that is us.
I heard Rabbi Frand give this Derasha in person, but I found it online, so here it is:
If a person had any doubts about whether what he was doing was right or wrong, and his car suddenly stopped and told him "Don't Go" (and not just one of those recorded voices saying "Your seatbelt isn't buckled...") -- would that not cause the person to at least stop and wonder whether he was doing the right thing?
We may ask this question even about a person who is not perceptive. But Bilaam was a wise person; he was a perceptive person. How would a perceptive person relate to his donkey talking to him?
Bilaam should have thought, "My strength is my speech. Who gave me that power? G-d gave me that power. The proof is that the same G-d who gave me the power of speech, just gave my donkey the power of speech! 'Who gives a mouth to man or who makes one dumb...' [Shmos 4:11] Where is my strength from? My speech is no bigger of a miracle than my donkey talking. It is the same strength from G-d."
What should Bilaam have concluded? He should have concluded that he was not using his power of speech correctly, and he should turn back. Is this not as clear as day? Is the message not clear? Should it not that make an impression? Yet it did not have any impact.
This is the lesson to be learned: how blind a person can be! When a person has some type of personal motive -- whether it is money or power or whatever it is -- a person can literally be completely blind. G-d can almost spell it out to him... G-d CAN spell it out to him, but he still will not see it!