Yes, the taped conversations took place in early 2001 and so Italian police and intelligence sources knew of their existence for some time. However, they have only recently been translated and given to U.S. authorities.
The reasons given:
The tapes were difficult to hear due to lots of background noise and the terrorists spoke to each other in a little known or little used dialect.
So the whole world didn't know after all. Even if the tapes had been translated, transcribed and given to us earlier there is still not enough information on them for anyuone to determine the type of monsterous act they were planning.