This audio is a sonification of the radio signal transmitted by ESA's Mars Express (in X-Band at 8.4GHz) on 28 May 2015 at 13:50 UTC.
It was transmitted over a distance of 2.54 AU, equal to 380 million km and took 21 minutes to make the journey. It was recorded by Bertrand Pinel (F 5PL), using second-hand signal analysis equipment and a surplus French government radio dish antenna in his backyard, near Castelnaudary, France.
The original radio-frequency recording has been translated into the human audible range (20-2000 Hz). You can find tutorials (e.g. https://www.youtube.com/watch?v=qNf9nzvnd1k) which help illustrate the correspondence between radio frequencies and human-audible tones.
In this audio recording, you can perceive a shifting tone reflecting the contribution of the Doppler Effect due to the relative motion between Earth and the Mars Express spacecraft, which happened to be close to the point of pericentre (closest approach to Mars) in its orbit (it reached this point just 7 minutes later).
The continuous background hiss is due to noise signals collected by Bertrand's receiving dish antenna, in particular originating from the Sun, as Mars and Earth are now in conjunction (i.e. on opposite sides of the Sun). As a result, radio signals from the spacecraft must pass past and through the Sun's extremely chaotic and energetic atmosphere, contributing a great deal of random noise.
Bertrand mentions that one of the problems in catching the European spacecraft's signal is the verdant trees growing in his backyard, which interfere with his antenna. We think the maple and chestnut trees, in particular, won't last long once he finds his chain saw.
Audio file credit: ESA/B. Pinel/CC BY-SA 3.0 IGO
http://www.esa.int/Services/Creative_Commons_Attribution-ShareAlike_3.0_IGO_CC_BY-SA_3.0_IGO_licence
