The recordings were originally prepared for the 9/11 Commission, but they were never completed or released by the time the commission released its final report in 2004.
The first sign of trouble comes at 8:13 a.m. when Boston Air Traffic Control fails to reach American Airlines Flight 11. Then flight attendant Betty Ong called from the back of the plane.
"Somebody's stabbed in business class, and we can't breathe in business, and um, I think there is Mace that we can't breathe. I don't know, I think we're getting hijacked," she was heard saying.
Then the voice of one of the hijackers was heard as he addressed the passengers.
"Nobody move. Everything will be OK. If you try to make any moves, you will injure yourself and the airplane. Just stay quiet," he was heard saying on the recording.
In one chilling excerpt, screaming and a shouted "Hey!" is heard over the radio as hijackers storm the cockpit of United Flight 93. That's followed by a strange, strained cry. Stunned controllers and other pilots discuss the sounds, trying to make sense of what they heard.
Puzzlement and frustration is heard in the voices of controllers, military commanders, and even pilots watching the attacks from the sky.
There is shouting and ringing phones in the background - the soundtrack, usually omitted from written transcripts, of a nation suddenly at war.
After Flight 11 hit the World Trade Center, tapes recorded the conversation among controllers as a second plane streaked past the window of a radar control facility on its way to Manhattan.
"Another one just hit the building," someone says.
Another person responds: "Oh my God."
And then: "Another one just hit it hard. ... Another one just hit the World Trade."
It's followed by: "The whole building just, ah, came apart."
Someone utters again: "Oh my God."
In all, 114 recordings were released stretching across two hours of the morning of Sept. 11, 2001.
Some original recordings from 9/11 still have not been made public, including military communications, discussions among White House officials, and the cockpit voice recorder from Flight 93.
The Associated Press contributed to this report.