Here's a few ideas the come to mind:
- You could use different frequencies to trigger different things using a bank of analog bandpass filters, each connected to a different ADC input.
- Rather than just paying attention to the absolute loudness of the audio (of either the full signal or filtered bands of signal), you could pay attention to the change in loudness of the audio. That way a gradually building sounds (like an orchestra string section crescendo) might trigger slowly changing lights, while a drum or an explosion could trigger a bright strobe flash. This technique could be combined with the previous one (of splitting it into bands) to let you respond to very particular elements in the music (like you could just pick out the cymbals out of a song with many instruments, for instance).
- As the previous poster mentioned, the arduino can't provide enough power to switch things like relays or certain transistors (like triacs, which you might want to use to switch AC for lights), so having the arduino trigger a driver circuit can be a good way to do this. You can do this with darlington transistors (which are just a couple transistors doubled-up to switch a big load), mosfets (which can switch big loads), or the easiest route is a chip with a bunch of these on them, something like this: Driver Array. Check out the "interfacing with hardware" section of the Playground for similar things. Also, you might look into opto-isolators as to minimize your chances of frying your arduino(s).
- If you run out of outputs, switch to a serial input driver array, or use a "595" and "shiftOut" calls to do the job.
- If you're looking for a good overview of electronics (including everything I've mentioned, like bandpass filters, drivers, etc.), I really recommend this book: Practical Electronics for Inventors. Silly name, but quality content to make pretty much anything you might want.
I hope that helps.
Cheers,
Paul