Two choices come to mind:

I built a device that had three analog filters (low,mid,high) and these modulated the brightness of three dimmer channels. If you had a bit of spare cpu time kicking around you could do a FFT to eliminate the analog filters.

Otherwise:
Get a low pass filter (analog or otherwise) to extract the bass energy and have the doof,doof clock a sequencer( aka chaser) to make pretty light patterns. Some lighting desks do this already. Makes the dimmer a lot more complex as you need a user interface so the user can enter his light patterns etc...

You'll need an 'automatic level control' up front to give you a constant signal level to the rest of the electronics. This invariably involves analog electronics. You'll need to filter the input before it gets to the a/d anyway- Nyquist criterion ( the a/d has to sample at least twice the frequency of the input signal otherwise high frequencies warp into lower frequencies and shit happens - so the filter knocks of frequencies past the half sample rate). Search the 'Net for circuits.

Whilst you're adding these features, why not add some levers, cross fades , memories etc!!!!