I recently started re-reading the Wheel of Time series and something right at the beginning struck me that I hadn't noticed before. In the prologue to "The Eye of the World" Lews Therin Telamon is somewhat deluded by the madness and having a conversation with a stranger he does not recognise immediately, Elan Morin (Ishamael).
At one point in the dialogue, after saying it will be time for the singing soon, the Dragon asks the following questions of Ishamael; "Do you have the Voice stranger? Do you know the Song?"
In the edition I have the words Voice and Song are capitalised; now I assume the song is what the Ogier and the tinkers are searching for and the same as the Song that Rand sings much later in the stories to make things grow. Please correct me if this is not correct or feel free to clarify.
But one thing is not clear to me, what is the Voice?
EDIT: Something to add to this from another re-read of the books.
When Rand first sees the male Choedan Kal in Cairhien (in book 2) his thoughts discuss seeing light pulse in the crystal and light seemingly singing to him. This suggests, to me at least, that while we don't know what the Voice or the Song are exactly; we can be relatively certain that they are directly related to the One Power (Saidin/Saidar).