The Silence

The Voice  describes an unknown, but powerful external force or group that forced the Doctor's TARDIS to explode on the date of June 26th 2010, and thus created the Cracks that would erase various things throughout the universe, and eventually the universe itself.

In 1580 Venice, Rosanna Calvierri said that through some of the the cracks, she and her race saw only silence and the end of all things, implying that the silence came from the cracks in time. She would later go on to mention that her race "ran from the silence" in fear, through a crack that acted as a portal to that place and time, before it snapped shut behind them. Following his defeat of Rosanna and her race, when the Doctor was preparing to leave, a mysterious silence fell over Venice. (DW: The Vampires of Venice)

When it first took control of the TARDIS, the entity forced the TARDIS to land outside of Amy Pond's house on June 26th 2010, and began to explode shortly afterwards, trapping River Song inside of it, where she was put into a timeloop as the explosion happened continiously at every point in time until the Doctor saved her. Although the entity behind the voice was never visible or named, it was heard to be speaking before the TARDIS exploded, saying: "Silence will fall!" (DW: The Pandorica Opens/The Big Bang)

Behind the scenes

 * Steven Moffat has confirmed in Doctor Who Confidential that the Voice will be featured in Series 6, with its identity possibly revealed.
 * The Voice has alternatively been refered to as "The Silence" by both the Doctor and Steven Moffat, implying that the thing itself may (as being only a voice suggests) be incorporeal, hence an external force.
 * The actor who portrayed "The Voice" is as-of-yet unknown.