The following video is a compilation of various footage suggesting that a bomb was detonated in the World Trade Center just seconds before the first plane impact. In addition to numerous eyewitness accounts, there is an audio record by Ginny Carr at the 6:20 mark that reveals a loud noise about nine seconds before a second, much louder explosion that was presumably the plane crash. However, if the first explosion was the plane impact, the second explosion could be debris falling down the elevator shaft since the time delta is within the range of plausible freefall times.
This is followed by video from a local television station which also appears to record an explosion while sound of the airplane overhead remains audible for several additional seconds. Note that sound travels faster through solid material than in air, so it is possible that the camera, resting on the ground, picked up the explosion from the ground before the sound could travel the same distance in air.
Nevertheless, combined with eyewitness accounts, both recordings deserve further study.