Kind of like Horton Hears a Who…
Sound doesn’t pass through the vacuum of space. Sound travels through the air as waves, and if there is no air to act as a medium for those waves to pass through, sound doesn’t travel. Interstellar space is largely a vacuum, but gas can still exist out there, even in areas that appear to be completely empty. In the case of Voyager 1, it’s been picking up the faint hum of plasma waves in the interstellar medium.
For more, check out this BGR/MSN story.