Normally I'd say something about how it's not a good idea to resurrect a thread that's over three years old, but at least you picked one that was relevant.
The most straightforward way I see to do it would be to use a peak detector, followed by a low-pass filter, followed by a comparator with some hysteresis. If the input signal is too low of an amplitude to use effectively with a basic diode peak detector, you can easily throw an op-amp amplifier before the peak detector. The output of the comparator will then just be a digital indicator of whether there's a certain level of signal on the input, and you could use it to drive a relay or whatever you want to switch. You can adjust the switching threshold by adjusting the voltage threshold of the comparator, and/or adjusting the gain of the amplifier if you use one.