The Astartes videos that have come out over the past 12 months or so have been amazing, especially the last one. However, there is a lot going on in the very short amount of time, so it can be a little tricky to piece things together.
In Part 4, the Space Marines face off against two Psykers, and in this video, some of the sounds don't really... match up. Specifically, when the bolter rounds explode, and the footfall of the sprinting Marines. Surely these should be far louder. Especially when you compare the roar of the Bolter fire, compared to the party-popper-esque snap of each bolter round that pops in the psychic shield.
And when the second Marine charges, it's already too late before the Psyker realises he's there. Surely, with the massive size and weight of a Marine in full Power Armour would really be a lot louder; loud enough to alert the Psyker to his approach much sooner.
My theory is that these two Psykers are brining out all the stops, increasing their powers in whatever ways they can in order to do their best to stop the Marines. Reducing the area of effect of their abilities, in order to increase the power of them; or shutting down their other senses in order to increase their concentration.
This would explain why, if "hearing" these sounds from the Psyker's perspectives, exploding rounds sound like party poppers and a Marine in full sprint sounds like a 4 year old trying to be sneaky.
Is this the case?