Story line: Sometimes characters don't have a voice (not speaking)
I too am experiencing the same issue.
this is not limited to story mode, some npc in the game, especialy scouts seem to be missing their sound too. There was a post about this here: https://forum-en.gw2archive.eu/forum/support/bugs/Personal-Story-Cutscenes-Bugged-NPC-Dialogue/first#post114057