Joseph Nobles of ae911truth.info points out in his post "Explosion on 'Seven’s Exploding' Video is Not Original," that the firefighter in the clip does not say "seven's exploding," but rather "the city’s explodin." After taking this into consideration and listening closely I agree, but as is admitted by 911myths.com on the page "A WTC7 explosion video," "The firefighter's location isn't too far from WTC7. If a bomb were to go off in the building then it's reasonable to believe they would hear it..." Also, after the explosion is heard the firefighter points in the direction of WTC 7.
At first "debunkers" tried to attribute the sound to an exploding fuel tank, but comparisons of the two do not equate, however, a linear shaped charge fits the bill quite well. Here is a great video from Debunking the Debunkers blog contributor Adam Taylor highlighting these and other points.
Now "debunkers" have moved on to questioning the authenticity of the sound. 911 Myths reveals that the clip first appeared in a documentary called "9/11 Stories From The City" and states that, "...Being a Hungarian version with their own voiceover: the original volume is dropped considerably, which makes it hard to tell if the explosion sound is the same."
Well, here is the clip. There is this thing called a volume control, turn it up real high, hell, go all the way up and play it a few times. Also listen to see if there is "no echo on the explosion" as a JREF forum member on Nobles' page states.
The clip also appeared in another documentary that aired on HBO, which just so happened to cut it off right before the explosion. This is circumstantial evidence that there is a loud explosion that they didn't want to show.
9/11 Myths points out that based on the position of the sun we can tell when the explosion took place, concluding that, "If the sound did occur around 7 hours before WTC7 collapsed, as the evidence suggests, then there's no obvious reason why we should make a connection between those two events."
However, there is evidence that the demolition of WTC 7 was botched and supposed to have been timed with, or shortly after, the fall of the towers to make it less obvious.
On the 9/11 Myths page another JREF forum member is quoted saying that the explosion video "...was probably shot between the times that WTC2 and WTC1 collapsed, or it may have been somewhat after WTC1 collapsed -- perhaps even during the collapse."