The Voice

Strictly speaking, the Voice wasn't even 'heard' as such, as its warning words were explicitly within Frodo's mind. Nonetheless Frodo thought of the source as 'the Voice', having no better description available.

An interesting element of Gandalf's account of these events is that fact that he 'sat in a high place' while he strove with Sauron (The Two Towers III 5, The White Rider). It's not exactly clear what the significance of the 'high place' might be, but it seems to suggest that Gandalf was actually on Amon Hen at the time of the breaking of the Fellowship.

It seems strange to imagine Gandalf concealing himself nearby while the momentous events of that day unfolded, but it's difficult to see what other 'high place' he might have intended. We know that he was travelling south from Lórien at this point, and Amon Hen was the only significant high ground on the western banks of Anduin. After challenging the Dark Lord, Gandalf states that he was utterly drained, which perhaps explains why he failed to take part in any of the events that followed.

After diverting the Eye of Sauron, Gandalf says: 'Then I was weary, very weary; and I walked long in dark thought.' (ibid). He doesn't actually say how long this took, but five days later he met with the Three Hunters in Fangorn Forest, so the 'long' here probably accounts for no more than a day, or perhaps two.