How to interrcept and read audio data as it is being output in RaspberryPi

Specifically, I want to write a program to read audio data that is being output to the analog jack on a RaspberryPi that is running Pi MusicBox for a led visualizer. Previously I had a Processing sketch that could run on your average computer, but I am less experienced with ARM. I've considered writing a Java program to do this, but I haven't been able to find any good documentation.

Any help much appreciated, thanks!