Actually, most of Bin Laden’s followers (the citizens, not the actual Al-Queda memebers) don’t have access to VCR’s, but they do have tape recorders.
Audio tapes of his speaches has long been the traditional method of getting the word out. Whereas videotape is directed as much at the west as at his own people.
The fact that this is audio rather than video gives it some credibility provided that the content also appears to be directed at the common people, rather than at the west.