US Northern Command is in charge of monitoring and defending the US, Canada, and Mexico, while NORAD (a joint US-Canada operation) is in charge of monitoring and defending the airspace of the US and Canada. Needless to say, they played a big, disputed role in the events of 9/11.
Our friends over at Government Attic obtained and posted audio of the NORAD/USNORTHCOM radio channels on that day. With their blessing, The Memory Hole 2 has posted these files to the Internet Archive, meaning that you can now stream them online, download them as a torrent, or download the individual files in MP3 or open Ogg Vorbis formats.
Internet Archive page containing all these audio files in various formats