When Ron and Harry use Polyjuice Potion to transform into Crabbe and Goyle in the second book, Harry's thoughts read, "... Then he realized that his glasses were clouding his eyes because Goyle obviously didn't need them — he took them off and called, 'Are you two okay?' Goyle's low rasp of a voice issued from his mouth."
This clearly tells readers that under the effects of the potion you take on both the vision and voice of the person you've become. But this isn't what we see play out in the films.
For one thing, in the second movie, Harry (as Goyle) keeps his glasses on until Draco questions it, insinuating that he doesn't get Goyle's vision from drinking the potion. That also creates a discrepancy in "Harry Potter and the Deathly Hallows - Part 1" when Hermione uses Polyjuice Potion to become Harry and remarks on how terrible his eyesight is.
Additionally, Ron and Harry maintain their own voices in the second movie, which is consistent with the characters who become Harry in the first "Deathly Hallows" film, but still inconsistent with the book's description.
The inconsistencies continue when you take into account that when Barty Crouch Jr. uses Polyjuice Potion to impersonate Mad-Eye Moody in the fourth movie, his voice seemingly does transform.