It has been argued that many cameras (both video and still images) reveal no flashes and no sounds other than the expected rumble of collapse.

To me, the rumble heard is reminiscent of tons of explosions going off, but to each their own I suppose. In post 32, KennyJC argues that tons of explosions going off would uniformly smash the surrounding windows and probably deafen some in lower Manhattan and that the explosives were silent when you consider that bombs are typically heard for miles. I have argued that many smaller explosive devices may have made it simply sound like the rumble of a building falling down.

The BBC said:
"there are more explosions further down the building...there was an explosion about 20 stories further down, i was at the base of the building when it happened, there was one huge bang..then 2 or three bangs"
https://www.youtube.com/watch?v=45fZlX1ZCU0

"there was another big explosion...then an hour later we had that big explosion from much lower...I don't know what caused that" watch his reaction as the second shockwave hits him. notice that the video sound recording does not pick up the shockwave:
https://www.youtube.com/watch?v=D17QP2kna1I

Here he is again describing many major explosions as the planes hit, after the planes hit and just prior to the collapse of the towers.
https://www.youtube.com/watch?v=Bjezhp0rL3M&feature=related

The other issue is the witnesses. The flashes may not have been to common a witness site, but the -sounds- and other evidence of explosions is quite pronounced:
http://911research.wtc7.net/wtc/evidence/oralhistories/explosions.html

In regards to the flashes, there is actually a fair amount of evidence:
http://www.sciforums.com/showpost.php?p=2074695&postcount=1962
http://www.sciforums.com/showpost.php?p=2074474&postcount=1942
http://www.sciforums.com/showpost.php?p=2074365&postcount=1928
http://www.sciforums.com/showpost.php?p=2031653&postcount=1039

As well as this video:
https://www.youtube.com/watch?v=8n-nT-luFIw