Don't forget that it's an inexact translation.
Why are people so quick to say "it's a phony!" everytime OBL releases a tape?
Those people are stupid, but not that stupid. Surely, they know that we'd detect a fake a mile away and that would be publicized. They wouldn't risk it.
Many thought Saddam's taped message after we first bombed Baghdad was a fake. WRONG. It was him. It's the same with OBL. We hear this "FAKE!" every time. How many times in the past have they been "fake"? ZERO.
Perhaps it is because some folks just want it to be so (that OBL is dead). With all the technology the world has today, and the fact that there are known audio recordings of OBL to use as a comparison when receiving a new audio tape, it is highly unlikely that our CIA, or any other intelligence agency in the world, cannot determine the authenticity of said person's voice.