Why can’t I upload an audio file that is larger than X MB?
This issue actually has nothing to do with this plugin. Your server and your WordPress installation will each have a maximum file upload size that you are allowed to make use of and there is no way for this plugin to override that safely.
This tutorial should help you with increasing the maximum file upload size.
Of course if you want unlimited, awesome storage for your podcast then consider using Castos hosting, and then viola, Problem Solved.