You normally don't use shielded cable for speakers but this may have been left over from an older "talk back" system. You need to go to each location and see how the speakers are wired. Black would go to the black lead of the transformer, the white to the tap lead. At the amp the black would go to (-) and white to the (+) but it shouldn't matter. It will work just fine either way.

What concerns me is that it apparently worked with the shield connected to the (-) and the white connected to the (+)? And when you removed the shield and connected the black in it's place you let the magic smoke out. So yeah, you really need to get at those speakers and wiring to see what's going on. Are these ceiling speakers? New or old? Old existing speakers in a wall panel?

It's unlikely that a short in the wiring would cause damage like that because amps today are pretty well protected. So I'm thinking that there is some kind of voltage between the black and white. Easy enough to check with your volt meter.

-Hal

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