I don't see how the previous got linked to a previous thread so I am reposting with additional clarification.
OK, I did some testing on an other machine with a testing installation, downloaded FMIT (an instrument tuner that will pickup audio input/analog and tell you all kinds of stuff about the wave that is fed). Same exact behavior, hardware only lists two audio inputs and are both unplugged. The lower the db threshold for sound recording the more apparent the sound becomes. Each knock on the box shows corresponding amplification of that noise. Ok, so I picked up 2-3 live debian based USB sticks I have. I plugged then in to 3 different machines, downloaded FMIT and run it. Similar results. So it has nothing to do with my configuration of stuff. I say if some software is picking up sound from your environment and records it when no inputs are plugged in I would call this a "security" issue not a malfunction. What is there MORE to post so it can be reproduced? Blank live debian installations adding FMIT in 3 machines Jessie + Stretch (one on UPS, one on mains-plug, on laptop) if you allow it to see a wide spectrum of frequencies it records noise (mostly hi-freq) and only hardware inputs are unplugged. Where is the noise coming from? It probably has nothing to do with the program itself but its dependencies and some code feeding in wrong data as audio input. But it is actual sound -- "The most violent element in society is ignorance" rEG