Best Reddit comments:

This isn't the sound a comet makes, this is the sound that Rosetta picks up due to variations in the magnetic field of the comet it's orbiting. It emits a magnetic waves that a radio can pick up that is too low in frequency to be heard by humans, so they altered the frequency to be audible for humans.

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It's not "sound", it's electron-ion acoustic wave interactions in the tenuous gas around the comet being ionized by solar wind and radiation - plasma waves.

Moving electric charges obviously radiate EM radiation which is received by radio antennas on the probe, then is amplified and pitch shifted into the audible region for humans to perceive.

The origin of the particular oscillation frequency in this data is unknown.

Reddit comments:

https://reddit.com/r/videos/comments/2mafk8/what_does_a_comet_sound_like_it_sounds_like_this/