Note that Humash didn’t come with a CD indicating how to pronounce things. The technology to produce a record player did not involve plastic or electronics, so in theory one could have made one 3000 years ago, and the fact that instructions weren’t included at Sinai suggests that “perfect” pronunciation is not a mitsva (and patach is a sound that varies considerably among kosher Yidden).
Also note that Purim is associated with consumption of alcohol, which is know to impair correct pronunciation.