The speaker is not 'speaker' before the act of enunciation. With enunciation, speaker becomes both speaker and subject; the enunciation positions him or her vis-à-vis the language, whilst at the same time that relationship shapes the enunciation.
This passage states Benveniste's central thesis: enunciation is the constitutive act by which a speaker is simultaneously produced as a subject, making it the founding event of subjectivity in language.
, Last Lectures: Collège de France 1968 and 1969, 2012thesis