The "unknown voice" is present on many of the Unfavorable Semicircle videos - the voice is male, not synthesized, and the only known voice to have been recorded onto any of these videos. There is reasoning to suggest the voice is British, although they say "Zee" rather than "Zed".
A very rudimentary microphone is being used - maybe a webcam, built-in laptop mic, or something similar.
Trends in Speech
On the first day of videos being posted, all videos were silent. The voice began to appear following after this, albeit far less frequently than it would later. It is remarked by /u/McSweepyPants that from here to 12 June 2015 that the voice appears to only, or at least mainly, say either "0" or "1" around this time. The same redditor later notes that from then until LOCK is posted, there is a good deal of microphone fumbling, potentially even heartbeat heard over this period of time..
Sample of voice
Videos Containing Voice