The "unknown voice" is present on many of the Unfavorable Semicircle videos. The voice is male and not synthesized (though presumably subjected to audio processing). The voice's accent has been subject to debate — some have said it sounds British, although it says "Zee" rather than "Zed".
The "lo-fi" quality of the sound suggests that 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 after this, albeit far less frequently than it would later.
It was remarked by reddit user /u/McSweepyPants that until June 12, 2015 that the voice appeared to only (or at least mainly) say either "0" or "1" around this time. The same redditor later noted that from then until ♐LOCK was posted, there was a good deal of microphone fumbling (or potentially even a heartbeat sounds) heard over this period of time.
Sample of voice
waveforms of "standard alphabet", as in the audio file linked above.