ScienceCasts: The Sounds of Interstellar Space

Tim Walker "Strictly speaking, the plasma wave instrument does not detect sound. Instead it senses waves of electrons in the ionized gas or “plasma” that Voyager travels through. No human ear could hear these plasma waves. Nevertheless, because they occur at audio frequencies, between a few hundred and a few thousand hertz, “we can play the data through a loudspeaker and listen,” says Gurnett. “The pitch and frequency tell us about the density of gas surrounding the spacecraft.” "

http://scitechdaily.com/voyager-1-presents-sounds-interstellar-space/