This is some code I used to test a couple of different methods of using the chip. One mode is 7 LEDs that vary in brightness to different frequencies. The other is lighting more or less LEDs depending on the amount of bass it's getting. (So basically like a beat detector, though less fancy.)
I've started using PlatformIO with Atom to upload my code. So the .cpp file is basically equivalent to an Arduino's .pde or .ino only with the extra #include <Arduino.h> at the top.
You should be able to get this project, open it up in Atom, then go to PlatformIO > Build, then PlatformIO > Upload.
It's written for the Mega2560, but can be modified for others.