This may not be as complete or as specific as you wanted, but I think it's the best we can do.
The voice was someone from the religious order known as 'The Silence'.
Just before we hear the voiceover, the scanner lights up and reads:
Location: Earth. Date 26/06/2010.
Then the scanner glass cracks.
The central belief of The Silence was that the question "Doctor who?" should never be answered, and that killing The Doctor was the only way to ensure it never was. To this end, The Silence made two attempts to kill, or silence, The Doctor. The first attempt was by locking him inside the Pandorica. After this failed, The Doctor noted that the Silence were still out there. Their second attempt was by using River to shoot him by the lake. On that second occasion, The Doctor allowed them to believe they had been successful in order to stop their campaign.
Everybody involved in these attempts could travel through time. The Doctor, River, and the Silence, who were apparently using the crack in space and time Amy found on her wall to do so. But The Doctor's death had to be a fixed point in time.
The first 'fixed' date on which The Silence hoped to kill the Doctor was the 26th June 2010. This is the date seen on the screen in the TARDIS when River lands and we hear the voice say "Silence Will Fall". This is the actual air date of the subsequent episode The Big Bang ('Pandorica' was aired the week before on 19/6/2020). The Silence had evidently manipulated events to bring The Doctor to this particular date in order to trap him. As River lands and the TARDIS screen shows the date, it cracks in the familiar shape of the crack in time and the voice speaks, indicating that The Silence is behind bringing them to this date.
The second attempt to kill the doctor was 22nd April 2011, one day before the airdate of 'The Impossible Astronaut'). We learn a lot more about The Silence in the second story arc, and it is only when applying this knowledge to the story involving the Pandorica that we can fully understand it.
