The voice is that of a soft-spoken 28-year-old, but the message is vintage Osama bin Laden, giving orders to kill. When the audio recording began turning up on jihadist websites two weeks ago, it was as if the dead terrorist was channeling himself through his favorite son. “Prepare diligently to inflict crippling losses on those who have disbelieved,” Hamza bin Laden, scion of the Sept. 11, 2001,...