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It's only possible to guess. So, my most likely guess would be that the outdoor speakers have simply deteriorated. If they were blown, they would most likely make some kind of crackling or scratching sound, or the high end would simply be gone. It you can take the covers off, you might do...

You’ll have to explain how everything is connected.

You said each zone has its own volume control. I would guess the outside has been turned down. At least, that's the most obvious reason.