These "sounds" consist of vibrations produced by the astronaut's movements and his suit systems and transferred through his space suit, camera enclosure and PCB the microphone is soldered to plus electrical and digital noise added by the sound processing pipeline.
Microphone is an electromechanical device, that converts vibrations into electricity. Different types of microphones contain different mechanical parts, that can easily perceive air density oscillations. Usually it is a sort of thin foil membrane fixed around edges and vibrating in the middle. Since its so lightweight, it picks all sorts of vibrations that reach it - taps of your fingers touching the enclosure and buttons, your steps, ultrasounds that you can't even hear - everything (that's why on Earth studio microphones are frequently vibroisolated using so called shock mounts). You don't need air to conduct them, because dense materials usually conduct vibrations even better than air (think about echolocation on the submarines). Also membrane sensitivity should be somewhat stronger in space, where the amplitude of membrane's vibrations isn't limited by the pull of gravity.
This membrane is one half of variable capacitor or a coil, which produces small oscillations of current in the input circuit of the microphone pre-amplifier, thus converting them into electricity. You need pre-amp, because those oscillations are really small, in the millivolts range. Pre-amp produces amplified signal suitable for analog-to-digital conversion and recording, but all the pipeline parts - amplification, ADC and sound compression - add their own noise to the microphone signal. That's why you hear something even when there are no obvious vibrations produced by the astronaut himself.