ANOTHER UPDATE: Nelson Ascher suggests that even if he's alive, the preference for audio tapes might tell us something:
Is Osama alive? We don't know, obviously, but the goal of releasing a tape is to use it as proof that yes, he is. The main argument that has been used against this kind of vocal proof is to say that were he among the living, he could as easily have sent videos instead of audiotapes. But, what if he has been badly wounded, having had for instance his face desfigured or some limbs amputated? Then he surely wouldn't want us to see that some kind of punishment has already reached him, not at least before his own revenge, right? Well, maybe this could explain the lack of images.
Yeah. Personally, I'm quite skeptical of these tapes. As a sound engineer, I could put together a genuine-sounding tape pretty easily if I had access to a lot of genuine old tapes.