Defiant Agents (Version 2) by Andre Norton.

Operation Cochise: a carefully planned Western move to colonize a planet ahead of the Reds. Travis never learned why the ship had crashed, nor why he and the other Apache agents had been shot into space without warning and under Redax control, a machine which had returned them to ancestral mentality. But the dangers on Topaz demanded free minds, for Travis soon realized that if the Reds already encamped beyond the mountains--a horde of barbaric Mongols completely dominated by their masters--discovered the secret of the eerie underground chamber in the towers hidden in a valley of mists, not only Topaz but Terra itself would be destroyed.

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Public-domain recording courtesy of LibriVox (librivox.org).

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