The Silence

The Voice describes an unknown, but powerful external force 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.

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.
 * As its actions nearly caused the whole of reality to be erased, it is likely the Voice comes from a completely different realm.
 * Going with the above, an entity refered to as "the silence" was seen through the crack, refered to as a who or what in Confidential, it is most likely that, from what the voice did and said, it might be "The Silence" entity. The Silence themselves seemed to have been refered to as a group or entity in Confidential, so it may be that it was the voice of The Silence.