U.S. officials told ABC News they could not confirm the authenticity of the audio at this time. The private SITE intelligence group, which tracks jihadi messaging online, noted the voice sounds like al-Zawahiri, and al Qaeda has generally not been known to fake messages from its leaders.
Mullah Omar's death was announced by Afghan officials late last month and the Taliban confirmed their vaunted militant commander was dead -- and said he died more than two years ago in Pakistan.
Until now, Al-Zawahiri had been silent on Omar's death and similarly had said nothing after al Qaeda's number two man and al-Zawahiri's own potential successor Nasir al-Wahishi was killed in an airstrike in June in Yemen.
Earlier this week Bruce Riedel, a 30-year veteran of the CIA, published a report online titled "Where in the world is al-Qaida's leader?" saying al-Zawahiri's 11-month absence was "unprecedented."
"So what accounts for the silence? Why is Zawahri missing in action? Of course, only he and his closest circle know for sure," Riedel wrote for The Brookings Institution. "Perhaps Zawahri is ill and his health precludes any public role. Like Mullah Omar, he may be incapable of activity or even dead. But there is no hint of that in the jihadi Web sites or chatter. More likely his silence is deliberate. He is biding his time and focusing on his own security. Al-Qaida has often chosen to out wait its enemies and go underground, biding its time."
"Zawahri knows the Americans are leaving Afghanistan next year; perhaps he is just waiting for them to go home," he wrote.