Rabbeinu Bechayei on Pirkei Avot 5:8 expands on the Mishnah citation that among the 10 things created during twilight prior to the start of Shabbat, is the mouth of the donkey that spoke to Bil'am.
He states that all 10 things in the list were created for the honor of Israel. The main idea to be learned is that the donkey doesn't speak on its own, but rather, that it is G-d who gives the donkey the power to speak. I.e., rather than viewing this as an unusual "miracle" and be surprised or "shocked" by it, Jews would understand that there is nothing too unusual or powerful for G-d to create, and a talking donkey is not unusual.
Take a close look at the wording of the Mishnah itself. It says ten things were created, not "ten miracles", as you have written in your question. Yes, we (not just you and I, but many of us) think of these ten items as "miracles" because we are not used to talking animals (i.e. - besides parrots & maccaws, etc. that, to some degree, can speak in "human" language). But, in fact, the Mishnah teaches us that these were part of creation, so at the point that it occurs, and when we read about them, we are not viewing them as unusual miracles, but part of G-d's plan.
Also, note, that this may explain why Bil'am doesn't seem surprised about his talking donkey. Perhaps he watched Shrek the movie, first ;-) Seriously, Rash"i, among others, explain that Bil'am was a prophet almost equivalent to Moses. So, granted, that this Mishnah was not written (though, it may have been orally transmitted by Moses, already), but, at the least, having a high, prophetic intelligence, Bil'am may have understood that a talking donkey was part of G-d's powers.
See this M.Y. question / answer explaining the significance of why these 10 items were specifically created before Shabbat.