It sounds like it indeed was a GoPro, neat! Were you not surprised to get video back, then? What steps were taken? I don't know much about Space Stuff but seems like even with shielding, you would expect off-the-shelf consumer hardware to fail somehow..

How interesting... the anacoustic zone apparently begins at about 160km altitude.

maybe a lot of that audio is electromagnetic interference? that's what it sounds like, and audio amplifiers can be pretty sensitive

also, sometimes audio amplifiers oscillate on their own (though i think the single-jfet preamp in an electret mic is unconditionally stable); an amplifier that oscillates in space in a way that physically vibrates the microphone might be damped by air so that you don't notice it when air is present

interestingly, microphone amplifiers oscillating and producing physical vibrations is not a phenomenon restricted to electronic systems: https://en.wikipedia.org/wiki/Otoacoustic_emission

pfdietz seems to be suggesting corona discharge encouraged by the lowered paschen voltage in low vacuum, and of course the negative-resistance characteristic of gas discharges can easily set up a parasitic relaxation oscillator; all you need is some parallel capacitance or series inductance, plus some kind of ballast that quenches the discharge http://tinyurl.com/23dt3d5e http://tinyurl.com/2de5uaog https://www.physics.purdue.edu/demos/display_page.php?item=6...