I have now installed one PC with cut scenes and one without then done a directory comparison between the two.
The one with cut scenes has two additional files: MOVIES01.MIX and MOVIES02.MIX. I will try copying these.
Also, a comparison of original CD media (GDI, NOD, and Firestorm disks) to my installation directories shows that Firestorm has a maps03.mix and movies03.mix but neither of my installation directories have them. I will try to copy those too, but I am not sure what to look for to confirm success.