As reported on Wired.com, linked to on a page with a story about Proximity Fuses on insurgent artillery in Iraq, which I originally discovered through AntiWar.com.

http://blog.wired.com/defense/2008/04/norad-releases.html

which links to:

http://www.governmentattic.org/docs/NORAD-USNORTHCOM_9-11_Tapes.pdf

The master PDF contains the following header:

Twenty-one 9-11 recordings from North American
Aerospace Defense Command and United States
Northern Command (NORAD-USNORTHCOM)

This material totals over 100 hours of audio recordings of
various NORAD/NORTHCOM communications channels
on September 11, 2001 in 21 separate mp3 audio files.

I don't have much experience with "GovernmentAttic.com", but their site has the following statement: