If there is a problem with the line picking up noise then you could add a relay to short the line when there is no audio

But that would require an extra pair. With long line lengths you would want the amp at the speaker muted. I think even with a short at the other end the line will pick up noise. A coded squelch would be a better solution but then you get into custom designed and manufactured equipment since nobody makes such an animal.

-Hal

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