The 2.0 development version has custom mp3 support now.
See this thread regarding 2.0 development:
https://community.smartthings.com/t/bigtalker-2-0-development/
Specifically this post regarding mp3 support: https://community.smartthings.com/t/bigtalker-2-0-development/55305/86
Note: 2.0 is a completely new install from 1.x. They can run side by side. It is also an in-development version. Expect it to break and provide feedback.