I thought about the possibility that the voice could be Shiva. Plotwise, it would make sense, but I hope it's Murtaugh. Murtaugh and Elizabeth are characters created for Submachine. Within the confines of the story a cultish religion was created to worship them. Shiva is an existing god in a religion that exists outside of the submachine universe. Having Shiva become a character within submachine either makes the story blasphemous or religous. That makes me a little uneasy, so I hope that voice is our first auditory interaction with Murtaugh and not Shiva.
I disagree with the people saying since we found Mur and Liz's tombs our interaction with them is over. We the player live within our own time, and it's been established that Mur and Liz have been dead for centuries. The only explanation I can think of for this is we've been interacting with sentient ghosts the entire series. Mur has said things and left notes throughout the series making it seem like we're seeing evidence of him at different points before and after his second enlightenment. I believe he is trying to use us to reach the core and try to reverse the chaos he created. Liz seems to have reached enlightenment before Mur, but as an inhabitant of the subachine she can't help him in the same way we can as outsiders. Shiva already showed Murtaugh the future and it's a happy ending, so we know we can't mess anything up.
My biggest question is what do people who have reached enlightenment actually see? They can't see people or details of specific layers unless they "lose focus", so what does the 8th layer look like? At first I thought it was along the same lines as "the 4th dimension is time", and didn't really exist in the same way as the other dimensions, but now I'm not sure. All the time warping stuff always confused the living shit out of me because I firmly believe that time travel is impossible in any iteration. I can't suspend my disbelief enough for it to even make sci-fi sense.
Mateusz must love reading our theories because we're always so far off the mark. I even venture to say he reads theories and deliberately avoids having the story resemble any of them.