Audio Input into A0?
Depending on how loud your audio signal is, you may be able to do it with just one capacitor and two resistors:



If your signal is quite quiet, though, you may need to amplify it using an op-amp. For instance:



That arrangement would double the volume of the audio. It also has the effect of inverting it, so you will need to invert your readings in software.

Also note that both of these methods will give you an AC waveform centred around 2.5V, so silence will be a reading of around 512, and it will go up and down from there. You can sample that audio waveform and do some basic (though quite slow on an Arduino) analysis of it, such as FFT to get the frequency content of the audio - great for visual effects...