Interestingly, the serpent/snake speaking to Adam and Eve is not the only instance in the Bible where an animal speaks. The prophet Balaam was rebuked by his donkey (see Numbers 22:21-35). We have to remember that while animals are not capable of speaking, there are powerful beings out there (God, the angels, Satan, the demons) who are capable of the impossible, including enabling animals to speak. Most scholars hold that it was Satan in the Garden of Eden who was speaking through the snake, not the snake itself speaking on its own. Thus, the Genesis 3 account it is not suggesting that snakes were of an intellect that would have enabled them to speak coherently.
Still, why didn’t Adam and Eve find it strange that an animal was speaking to them? It is unlikely that Adam and Eve had the same perspective we do on animals. In our era, we know from experience that animals are incapable of speech on the same level as humans. Adam and Eve did not have a childhood, nor did they have other humans to learn from. Given that Adam and Eve had probably only been alive a matter of days, it is not unreasonable for them to believe that animals were capable of speech. It is also possible that this was not the first talking animal Adam and Eve had encountered. Perhaps Satan or even God Himself had used animals to communicate with Adam and Eve before. There are so few details given in the account that much is left to speculation and presumption.
Lastly, it was not unreasonable for Eve to answer the snake. After all, the snake was evidently speaking in a language that she understood and asking an intelligible question. It is also likely that Adam was nearby and could verify that she was not imagining things. It was not the serpent speaking that should have alarmed them. Rather, it was the fact that he was causing them to doubt God’s instructions (Genesis 3:1), contradicting God (Genesis 3:4), and calling God’s motives into question (Genesis 3:5). That should have been enough to cause both Eve and Adam to stop talking to the serpent.