It seems absurd to suggest that a voice recording of firefighters is relevant to the questions of whether ZM is guilty.
I was wondering how they will tie it in as well. Could a terrorist have survived up there and his voice heard on the tape (I don't think so)? That thought is beyond a stretch, but I am not sure the significance to the trial either, since they have enough other info to hang him and others.
Aside from that, it just confirms and extends the heroics of these brave, selfless Americans.