Circuit to put digital amplifier into standby when no audio is at inputs?
I have implemented something similar to that described by @Dean to turn on/off my whole house audio system when audio is available.

I used an LM339 dual comparator.  I AC couple the input signal to the first comparator with the other input set to a DC voltage representing the smallest signal I wish to turn the system on. The open collector output is arranged to go low when signal goes above the threshold.  There is a weak pull-up and a large capacitor to give a time delay of a few minutes.

At the output of the first comparator then goes to the second comparator so that when if the voltage on the capacitor is below a threshold the output signal then enables the audio amplifiers (in my case it actually switches the power supply).

A few minutes after the audio input goes away the amp turns off and it only needs a short duration of signal to enable the amplifiers.

kevin