In the USLaunchReport video (see link below), there is about a 5.5 second delay between what is seen and what is heard - the camera is about 1.5 miles away. About 5 seconds before the report of the initial explosion, there is a very large bang sound, like someone striking a garbage pail with a hammer. The sound clearly originates in the distance, something that is made more likely by the fact that the launch site and surround would have been basically abandoned for a live fire test.
Now, visually, there is no event at the launch site that corresponds to this loud sound. It would have come at about 1:06, 5 seconds before the explosion. What could make such a large noise yet leave no visual record?
Some time ago, the SpaceX Falcon 9 CRS-7 mission failed when, just as in this case, the 2nd stage LOX tank failed during flight. The helium pressurization for these 2nd stage LOX tanks is derived from a smaller vessel that is actually housed within the tank and secured to it with steel struts. When the LOX tank is full, there is a large buoyant force on this vessel, like a football held under water, and this puts great stress on the support infrastructure. The in-flight failure was caused by failure of the securing struts, at which point the now-free helium vessel rocketed (ha) to the surface of the liquid oxygen pool and slammed into the LOX tank wall, fatally weakening it.
It seems possible that the loud noise heard on the video is the audible report of the same event. 5 seconds later, the tank fails.
https://www.youtube.com/watch?v=_BgJEXQkjNQ