I've been involved working with audio for years, some pro stuff, but mostly as a hobbyist sound guy and musician. So this topic caught my eye as these kind of things interest me.
I looked into the audio. The evidence seems inconclusive. Proving a negative, that the audio is NOT altered, is impossible in this case without other evidence (e.g. knowing and trusting the chain of custody of the tape, hearing the original audio). However, I can't find any clear evidence in the audio that the sound of the explosion has been added in later or even altered in any apparently suspicious way. Nothing in the amplitude curve or frequency graphs seems to clearly differentiate the explosion from other sounds heard in the audio nor it sounds intuitively artificial to my ears.
I THINK that the explosion is real. The fact that after the event the firefighters are talking about explosion would suggest that there really was an explsion. It has been eshtablished that on that day there were plethora of things exploding (cars, gas, ect) or making sounds that could perhaps be interpreted as an explosion in urban setting pruducing these huge echo tails (various stuff breaking, falling, colliding ect.) So one should expect to see and hear videos like this.
However I found three strange events in this short piece of audio i looked into. On the other hand one could say, that one would find unexpected events and artifacts in ANY piece of audio if one looks close enough. Two words to google: dictabelt evidence.
1. seconds before the explosion there is a short period of silence. This kind of event is not seen anywhere else in the audio. It is impossible to say what caused this short period of silence. It could be an artifact in the recording or broadcasting stage itself, or it may have been caused in the process of editing the audio. For example it could be a mistake by someone using fade-in/fade-out tool for some reason thus causing the short period of silence. Using fade-ins and -outs would be expected if someone was altering the sound (for example adding something on purpose). If this is the case, it could be compared to making a small mistake with photoshop leaving few pixels out of the "magic wand" or clicking and altering some part of the picture by mistake and not pressing ctrl+z afterwards. Of course, one can make these mistakes without trying to add or hide something from the audio. See below.
2. After the explosion event, as the echo tail of the sound is fading, there is a drop and rise in the overall volume of the audio. This can be explained by "natural causes", as the recording device or the signal chain in broadcasting stage (if this was live?) would most probably have a function called "auto gain". It automaticly tracks the overall volume of the incoming signal and adjusts microphone/signal gain accordingly. This process kicks in at certain amplitude/time threshold. There could also be various limiters and/or compressors on the signal chain somewhere that could perhaps in certain settings cause something like this. However they are unlikely to produce these kind of results. This again however could also be an indication of someone using various editing tools when editing the audio for some reason. It is quite common to manually edit the volume slider, making sure that the loudest voices are not too loud compared to other sounds in the signal or vice-versa. In most cases editing the audio in various ways to make it sound clearer/better/more pleasant is an integral part of publishing process of almost any video made by any serious producer, just like adjusting the overall colours, contrast ect.
Ok, as I was just posting this I watched the video again. I think this is propably just be the microphone moving as the cameraman is turning around. My bad!
3. Weird click. This is propably most dubious thing on this video. This event occurs RIGHT at the moment of the firemen saying something that one perhaps could interpept as "building seven". This is very odd and fishy indeed. One should hear this click even with laptop speakers. It is however very hard to say what this saund or it's source is. It seems to be quite sharp "TICK!" -sound. Something that one could perhaps expect to hear from a ramdomly programmed drum synthesizer. This kind of sound is NOT heard anywhere else in the audio. The frequency print of this sound would suggest that it is not something just happening in the background. It could however for example be a result from something physichally hitting the microphone thus producing a sound that would not seem to fit in. Given that this event is right at the "punchline" of the whole video, one can suspect some trickery here. However, in my opninion, even here the audio evidence alone is inconclusive.
And finally I have to say that the audio quality of this clip is very poor, and all these events and artifacts really hold no real evidence, as they could be explained by just crappy compression and editing ect. It's a long way from the recording device to stupid youtube video - could be compressed who knows how many times by people who really dont know what they are doing, each time degrading sound quality, adding and amplifying artifacts ect.
And by the way. Listening the audio with a professional audio gear and headphones I would say that there is no mention of "building seven". The strange "TICK" however makes it difficult to interpret and I'm not a native speaker, but I think he is saying "serious explosion".
Few pictures as attachments since I can't post links.
