It's not impossible conditional on the existence of strong magic. I'm not so sure it's not still ridiculous even conditional on the existence of strong magic. Especially as, in the story, the snake doesn't appear to be magically talking, it's just "more cunning than any of the other creatures YHWH had made" or whatever exactly the text says.

We now know that talking requires a big fancy brain, such as humans have and snakes conspicuously don't (and don't have room for), and the right sort of vocal apparatus, ditto. Back in 2000BCE or whenever it was, of course it was well known by observation that snakes don't talk, but it presumably wasn't understood that they can't and why. And when we see an old story featuring a talking snake, which doesn't present it as able to talk on account of some sort of magic or divine intervention but just oh, hey, a talking snake, I think it's reasonable to say to ourselves "See, the people who wrote that story just didn't understand how implausible that bit of it is". And I think it's reasonable to see the talking snake as making the story less plausible than it would have been without it. And also, I think, less plausible than if its talking had been explicitly explained by magic or divine/diabolical intervention. Not because those are plausible explanations otherwise, but because the rest of the story is already committed to the sort of world in which such things might work, and in such a world a magically or divinely talking sheep is more plausible than an "ordinarily" talking sheep.