I've tried speaking to that demo several times... I used the built in feature to record from microphone, and I played back the samples to make sure they were audible and clear.
Sometimes it outputs the words "thank you" (which I did not say), sometimes it outputs a period. It never once output anything I said. It seems completely broken.
EDIT: apparently something about the combination of Safari+HF+Whisper was not working. I tried another Whisper demo on HF and had the same results. Switching to Chrome made it work flawlessly... I have no idea what kind of codec incompatibility was happening.