Audio levels
I do know about cameras and audio equipment. Critically, the camera here is mounted INSIDE an armoured vehicle. You can see the armoured glass of the windscreen. 7mm armoured glass cuts out pretty much all sound.
Then there's the matter of the recording equipment. The mic on a GoPro or similar dashboard cam will be set to automatically adjust sound to the same level all the time. So the driver's breathing and dog whimpering are recorded at the same level as the detonation. And as mickjoebill remarked there is no supersonic shock wave from a fireball. So no sharp "crack". You can hear the engines surging on the aircraft as it falls, then the automatic level control "clips" the audio level, and by the time of the impact the levels are reduced almost to zero.
I would expect that the decibel level of the surging engines at this distance would be about the same as the decibel level of the explosion - 130dB? Something like that? (that's a guesstimate, by the way). So the audio rec circuit would have already compensated for the increasing audio level as the aircraft fell.
In addition, as noted, the audio record quality is notoriously poor on a cheap camera. You can try it yourself on your camera phone - alternately whisper and shout and see what you hear.
The audio you hear when you watch a movie is totally unlike what is recorded by the microphones in real life. It's all overlaid, edited, with multiple layers of sound added to make the explosion sound more "real'.
I've filmed incoming artillery rounds, and a B-52 dumping a full load on Serbian positions in Kosovo, and land mines detonating, and IEDs; and even with full professional kit, when you play back the recording they sound fake and tinny.
Another point - No one gets into Bagram without being in some way connected to the military. From the little evidence we have, this guy is almost certainly a Brit with the K9 patrol unit. He will probably be an ex Royal Military Police dog handler, and he will know how to react to critical situations, having almost certainly served with the Army in Iraq and Afghanistan. He's almost certainly seen plenty of stuff go bang, and seen much more graphic and disturbing sights.
He kept his cool. That's all. Good man.