I doubt very much whether these tapes are bogus. Consider:
1. They were "delivered" to Al Jazeera, not to any other news source. I would think that Al Jazeera has ways and means to know that they're from an authentic source - these must be in place, or they could be hoaxed by anybody! Just as the IRA had "code words" to let British news media and police know that a bomb warning from them was authentic, so Al Qaeda must have ways to let Al Jazeera know that the tape they receive is authentic.
2. The US security establishment has a pretty awful record of keeping "dirty tricks" secret. I doubt very much that it would be possible to conceal an operation to produce bogus tapes like this, given the current political climate and the availability of "whistleblower" protection for informants - not to mention anti-war Senators and Congresspersons who would be delighted to offer protection and immunity to anyone testifying in front of their committees.
3. The timing of the tapes doesn't coincide with any particular legislative agenda. The Patriot Act renewal is an ongoing process. To have a bogus tape produced now, rather than a few months ago, when it was up for debate in the Senate, makes no sense.