The Speaker of the Borg Collective was a title held by those Borg who serve as 'diplomats' and as the primary form of interaction with another species. (DS9 - Millennium novel: The War of the Prophets)
Encounters[]
In 2366, Locutus was created when the Borg assimilated captain Jean-Luc Picard to act as envoy to the Federation. This was the first instance where a speaker was used to facilitate assimilation of a culture. (TNG episode: "The Best of Both Worlds")
In 2371, a Romulan dissident faction was responsible for forming a secret alliance with a branch of the Borg Collective with the intention of destroying the United Federation of Planets. To make the alliance as efficient as possible, they assimilated a Romulan who became known as Vox to serve as the Romulan Speaker of the Borg Collective. (TOS novel: The Return)