This issue was discussed at length some time ago in the following thread...
http://www.pprune.org/forums/showthr...threadid=50518
It's not entirely clear why the prosecutors need to play the tapes. Did the hijackers perhaps say something audible that could help establish that Zacarias Moussaoui was a co-conspirator?
I would have thought that the prosecution could have just entered the transcripts into evidence. Then if the defence wanted to dispute some of the transcription details they could have perhaps insisted on the tape being heard - but I very much doubt that the defence would want the tape played in court.