I just watched that last week. Liked it. It seemed like they had silent footage and dubbed in sound, or maybe there was originally sounds but stuff like BBC's The Great War (I swear I'd seen quite a bit of that footage in B&W elsewhere) it seems like mostly voice-overs and soundtrack vs audio and video from the same exact capture?
oh, man...yeah, you need to know the backstory going in, IMO....
Peter Jackson's team took all this saved footage from the British WWI archive and
digitized, colorized, overdubbed it with recorded interviews from people (since passed)
who had fought in those theaters and then employed a bunch of lip readers to recreate
the audio and dialog in the silent video footage.