Suggestion as a possible fix for this bug: https://taleoftwowastelands.com/content/protectron-missing-voice
I think robots in DC should have their own generic template as DC Raiders and Super Mutants do, since there are quite a few robots using the NV generic templates that point to the NV voice collection which don't have the quest/event specific dialogue for DC robots without their own individual creature entry.
This is probably a big change to TTW itself (since pointers in the Fallout 3 ESMs have to be modified) so this should probably wait until like 2.8 or 3.0.