'And when (it is said that) there (in the Sushupti) he does not hear, yet he is hearing, though he does not hear. For hearing is inseparable from the hearer, because it cannot perish. But there is then no second, nothing else different from him that he could hear.

In plain words: In deep, dreamless sleep a person hears nothing. Still, the power to hear stays and cannot perish. There is just no second thing there for him to hear.