So after watching the last Point of Interest: Episode 33, something came to my attention that I dismissed when I played the recent HoT Beta.
While playing a revenant, whispering voices is something you will hear very often. However after a while you barely even notice it.
When you channel the spirit of Shiro and use a healing skill, sometimes you will hear him talking to you. Asking you to set him free and let him go.
The dwarf king will keep shouting as he did before he had this meeting with fate.
Glint, as you are the Herald, is interesting, though. She keeps talking to you even without the use of skills and you can hear some "crystalline music" while you are channeling her.
But here comes the best part.
Since Mordremoth is calling out to the Sylvari to join his ranks, the player will hear him too.
Of course only if you have a Sylvari character.
This piece of information was given during the last POI. So I searched the web for a bit and found some part of the sentences you'll hear from Mordremoth.
Sylvari characters hear Mordremoth's voice in story missions and other races don't.https://www.reddit.com/r/Guildwars2/comments/3g286v/spoiler_about_sylvari_characters_in_the_hot_demo/
At least that's my theory. There were two situations in the story mission with a deep growling voice in the background. In the first "miniboss" fight when the boss calls out for help and the Mordrem Breacher appears, the voice actually answers with "I hear you my servant". Afterwards, when the attack on the camp starts, it says "You cannot fail!" (or something similar). In both cases the text also appears in chat, but without a name.
I couldn't check if other races hear those lines too in this version (and sadly I didn't find anyone on the twitch streams who played the story with a non-sylvari), but I'm like 100% sure that it wasn't there when I played the story in the closed beta, when only humans were allowed.
Edit: You can hear the first line here and the second line here (Sputti is talking over it though, so I can't really understand the text), thanks to /u/Mesurek for pointing to the right VoD.
It would be cool if someone could confirm that other races don't hear him!
So we can hear "I hear you my servant"
and "You cannot fail"
If any of you got more information on this topic - feel free to put it in the reply on here.