Audio Help

I need to be able to have the Arduino read the LF+, RF+, RR+, and LR+ speaker leads from a stereo and be able to tell me when there is audio on each line. I'd like to use an LM393 or such to build a circuit to trigger an opto-isolator attached to a digital I/O pin on the Arduino that would go HIGH with signal and LOW without. So far my circuits haven't been up to snuff. Any simple schematics or ideas to perform this task would be appreciated. Comparator or op-amps are not a must, just what I've been messing with.

Thanks! Doug

Is your amp a bridged amplifier internally?

This is a test unit for boat stereos. 4 channel with floating ground. No external amplifier.