I believe that it didn't change in the books, but it did in the movies. I think the writers just wanted to make things harder for Harry and Ron.
Thank you for that, Maia. Now I can say that the person's voice changes, too.
Thank you to everyone that has contributed!
If anyone else has anything they would like to input for others who made need this information, feel free!
There was a discussion early on in this term's NEWT Potions class where Polyjuice was discussed in a fair amount of depth. Here's the link to the thread - Link