Crude way would be to get a low pass filter and then detect the envelope of the audio signal on it.
Google for color organ schematics, just be aware that if it goes after the volume control then it will only work when you turn the control up loud. There are heaps of schematics out there since its a common thing that people want to DIY. There are a few ghetto ones on instructables, but I would suggest you avoid them like the plague.