On the Goblet of Fire Barty Crouch exactly got Mad-Eye's voice when he transformed into him, but on the Deathly Hallows, they were all able to have their own voices instead of Harry's. Why is the effect of the polyjuice potion inconsistent?
In that The Deathly Hallows scene, no mention is made of a change of people's voices, but later on the trio takes on the appearance of Ministry workers and their voices do change.
“Looks like it,” Harry whispered back; he voice came out deep and gravelly. (...) “Yes, of course,” said Harry in Runcorn’s deep voice.
Chamber of Secrets also clearly states that the voice changes as well as a person's appearance. Which makes sense, since it's their biology that is altered which would include their vocal cords. Speech patterns though (such as Fleur's accent) remain.
Therefore, Barty Crouch Jr. did not need to disguise his voice as he was taking polyjuice potion.
It's simple: the books and the movies follow different rules for the potion. This is from the differences in the media used. In the books it is easy to remember who's who as the author can keep telling you. In the movies however they need to remind people, which is why they had the voices of the ones drinking the potion.
So to clarify:
- In the books it changes the voice as well. Maybe not accents however.
- In the movies it does not change the voices.
The Mad Eye thing is because he looked like Mad Eye almost the whole movie so they just left it as the actor's voice playing Mad Eye.
Simple fact: Because of the audience and the difficulty it would be for them to distinguish the characters since in the book the author can distinguish transformed people. Since the movie cannot, keeping those voices is an easier method for the audience to avoid confusion.
Barty Crouch Jr. as Mad-Eye Moody was a special exception to the rule, a. his identity was kept secret until the end of the film, b. since he was not seen transforming, there was no need to change his voice and was a better method to conceal his identity.