How can I detect that sound is being generated I wish to use a mac mini as a home theatre system.

I would like the mini to send commands to the power amplifier to turn on/off depending on whether there is any sound generated (like a screen saver but for sound).

Anyone have ideas on how this can be done?

UPDATE:

Power Amp is Cambridge Azur 650R, which can be controlled by IR or RS-232.

Amp manual says not to keep on at all times, and I wish to conserve energy.

I am looking for something equivalent to the 'Sleepwatcher' utility (for the display, that I will use with my projector), but which works for sound.

I have found that the HALLab utility, from apple's XCode developer kit, can show that sound is 'Running somewhere', so it's possible in software.

Anyone know of a tool that uses this? Or which library calls I could use to do the same thing as HALLab?
I don't know of any way to react to sound output in software, but:

This could be done as a stand-alone hardware device reasonably straightforward. I didn't find any ready-made products, but it wouldn't be too hard for any electronics hobbyist with some analog experience, I think (here's a design I found, for example; it could probably also be done with a single microcontroller, especially if the amplifier has an IR remote control). Let me know if you'd like me to expand on this possibility...