This clip is the actual FAA Air Traffic Control transmission from ATC Cleveland Center on the morning of September 11, 2001. In addition to routine discussion, between the Center and pilots in the air, is the hijacking of United Flight 93.
Air traffic controllers, at Cleveland Center, ask other pilots to verify whether they heard screaming on a transmission from an in-air flight. Several reported they did - on two separate occasions.
Meanwhile, Cleveland Center continuously tries to make contact with Flight 93 - to no avail.
Near the end of this clip - as controllers try to figure out what is happening with Flight 93 - we hear someone say there is a bomb on board.
Clip online, courtesy FAA.