Narrowing my search re: signal sensing

What audio level?
What is the output impedance of the audio source?
What are the specs for the relay you want to drive?
What power supply do you have to run this?
What is the relay switching?

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Another question:
Do you need the relay to stay closed if there is a short break in the sound?

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LRG series - "Creative Applications" Smart Strip (Custom Build)
Another new model of the Smart Strip series is the LRG. When you're using the Smart Strip not just to plug stuff into, but as a piece of something you're creating, we've come up with a nifty feature. A small standard 2 wire connector on the side of the Smart Strip that when you short the connector together, it turns on the switched outlets; unshort it & it turns them off; for the hobbyist, it's a dream come true. This control signal circuit is similar to that which modern computers use which is a low voltage, low current signal to control your 120 VAC high current devices. Remember, however, that the control outlet will also still work exactly like any other Smart Strip so you can be creative in how you use this new feature. As with any other Smartstrip, the outstanding surge protection and power ‘scrubbing' ability will extend the lifespan of every electrical device plugged into it.

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Looking at the performance of its analog output, the maximum output level was 1.53V, 2.3dB lower than the CD standard's 2V, and the AE inverted signal polarity. The output impedance was a usefully low 15 ohms across the audioband

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