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It's actually the sound of the radio waves emitted from radiation. NYT has a pretty good write up on the matter. https://mobile.nytimes.com/2017/05/03/science/nasa-cassini-sound-recording-saturn.html

Sound is energy passing through a medium ( air, water, etc.) Because there is no medium in space , only vacuum, sound can't travel. But there are plenty of other energies running through space, so it's no surprise the rings emmit waves.