Max is the son of Goofy and the protagonist of his own movie, "A Goofy Movie". Much more calm and reserved than his dad, Max is a chill person who tries his best to help those he cares about. While he is only a side character for most of the comics, he is Yakko's best friend, and has some prominent roles in certain parts of the series. 

As an official toon, Max has a voice already in store! The entirety of "A Goofy Movie" can be used for inspiration. Max himself is pretty calm and casual when speaking, not overly energetic like most toons. This should be reflected in your performance when auditioning. Here's a clip of Max speaking as reference, but there are plenty more to go off of than just this one!
https://www.youtube.com/watch?v=MLnxJVP0E7o&t=6s