This would be a great place for a real NYC fireman to step in and tell us how & where the recording of the radio transmissions is usually handled. I'm tempted to guess that the recording setup is in a truck or van (remembering all the crushed vehicles at the periphery of the site), and that the vehicle might have protected the equipment to some extent. I don't see how it could have survived if it had been inside either of the towers or inside #7. Tape melts pretty easily.