What you probably want is a fast Fourier transform. This changes the sound into frequency information. There is a tutorial.
http://www.arduinoos.com/2010/10/fast-fourier-transform-fft/
It's probably more math than you were thinking of, but it will give you amplitude by frequency. Makes a good light show.
The other way you could do it would be with an electronic crossover. That way it's done in the hardware. But, doing it all in the Arduino would be a more elegant solution.
Of course, since you posted this before Xmas, you probably don't care any more.