This Speaker Power Detector might be a project that you want to add to your existing stereo system. The author is working on a revision so it may be something to do in the near future if you don’t want to implement the prototype design.

“The circuit revolves around the LM339, a low-power comparator designed to be easy to operate from a single (positive) power supply. A comparator compares two voltages and outputs a high, positive voltage if the first input (”non-inverting,” marked with a +) is higher or a zero or negative voltage if the second input (”inverting,” marked with a -) is higher. The circuit design then becomes pretty clear — just some connecting parts to hook the speaker wires up to the LM339 and see whether there’s a signal on them larger than some preset threshold.”