I can confirm that this issue is occurring to me as well. In fact, I can even reproduce it multiple times.
I rolled a brand new Charr Mesmer, Ash Legion. I’m level 3 and on the story step “The Tribune’s Call”. When I watched the cutscenes, certain lines of dialogue have no accompanying audio but the character will still make gestures.
When I exit the game in the instance of my story step, it will reset it the next time I log back in and I’d have to start over. When I do start over, the exact same lines of dialogue are missing their audio every time.