Thanks! Y’know, I think I’ve seen this and forgotten all about it. Didn’t he have a lip reader who constructed some of what was being said on the silent footage?
https://www.imdb.com/title/tt7905466/
Yep, that’s the one. Professional lip readers to gauge what the soldiers were saying, and professional voice actors to recreate their speech. Combined with colorizing the old footage it is a very impressive work, with full-color authentic WWI footage complete with sound!