Reference was posted on June 22, 2012.
It has the description "do not fail".
In total, this clip is 0:24 long.
The words "LOOK TO THE ORIGINS" appear. Tim's rant from Entry #59 plays, at high speed and distorted as an image of a tunnel is seen. A mask emerges from the darkness, the words "TRUTH WITH LIGHT" and a series of numbers appear.
Main Theories and Observations
Preliminary discussion on TV Tropes (and carried over to Unfiction) postulated that the numbers, when translated to letters in pairs, read "SL HE DA HI NT IE RK SI". Keeping the letter pairs together, this could possibly mean "HI SL IE SI NT HE DA RK" or "his lies in the dark."
Adding the individual numbers together (Example: 1912 -> 1+9+1+2=13 -> 1+3=4), results in "D D E H G E B I"
which, when rearranged, reads "He did beg."
The audio may also contain some speech from Alex from Entry #47.
Assuming that "His lies in the dark" is the intended meaning of the number puzzle, this could potentially be TTA referring to Tim. ~Zarggg
And seeing that Tim's mask emerges form the dark, so we see that he's lying about tha amnesia.
And in Reminder, TTA says "i have not forgotten" so we could probably say that Tim and Masky/TTA are different personalities with the same memories. Who knows if Jay also had his own proxy personality when had his 'amnesias'? Who knows if Jay was wanting to hide the truth with an amnesia excuse? TTA himself said once that Jay is "human, therefore [not trustable]"
LOOK TO THE ORIGINS (of Tim, or the Operator)confirmed, it's Tim
TRUTH WITH LIGHT (It can only be seen in the light, so it's a text)
HIS LIES IN THE DARK ("Lies" doesn't refer to "Origins" bacause it's plural, as the background speech is "get out of my life", "Lies" refer to "life" so "Tim's life lies in the dark")
HE DID BEG (TTA doesn't want Jay to disturb Tim again)
Answer: The Truth is in Tim's medical report
LOOK TO THE ORIGINS: Tim is Masky.
TRUTH WITH LIGHT: Tim is telling the truth.
HIS LIES IN THE DARK: Masky is the one who knows.
Basically pointing out that Masky is a alternate personality that Tim cannot tap into.
Since this video is most likely referring to Tim who many people think is Totheark, we can assume from this that Tim and Masky are not Totheark.
From what I've seen Tim has been in contact with The Operator for some time. Longer than even Alex. Coughing is a sign of being in contact with Slender Man. Tim was coughing during the filming of Marble Hornets and the blood in the sink when Jay went to the house was from Tim coughing it up. Alex was seen later coughing and spitting into his sink. "He did beg" could be an indication that he tried to fight The Operator's influence or at least begged him to stop.
If you re-arrange some of the words you get:
Truth lies in the dark.
Look with the light,
to his origins.
Although I admit it's a bit of a stretch. A more likely version simply re-aranged the sentences:
Look to the origins (Of Masky, or Slendy)
His lies in the dark (read like "His origins lie (or are located) in the Dark")
Truth with light (The truth will illuminate the mystery)