We don't know what Mars sounds like. It's a fact that most people don't know, and it's something that may change with NASA's newest rover that's the successor to Curiosity
There have been several attempts at a microphone on Mars, including numerous unsuccessful proposals and one failure with the Mars Polar Lander. Now, though, researchers are working on sending a microphone to the Red Planet to see what it actually sounds like in the depths of space.
With that said, the endeavor won't be easy. The atmospheric pressure on Mars is between 6 and 10 Torr. That's compared with the 735.56 Torr at sea level for the atmospheric pressure on Earth. This means that sounds will need to be a lot louder for human ears or microphones to actually hear.
With that said, even the thin atmosphere on Mars supports the propagation of sound waves. The microphone, though, will have to be powerful in order to capture these sound waves.
A microphone wouldn't just be used to capture sound, though. It would contribute to basic atmospheric science, such as gathering wind speed statistics, convective vortices, dust devil studies at close distance or when interacting with the rover. When used in combination with other data, such as visible imaging or meteorological measurements, sound could aid in the identification of many aeolian phenomena.
Needless to say, a microphone could be a huge boon to better understanding Mars - so why hasn't there been one before now? There were several missions that involved sending a microphone, but many of them lacked or lost funding. And NASA's Phoenix Lander actually had a microphone, but it was never turned on.
Now, scientists are planning for the new microphone to be incorporated into the next rover. The microphone will have to record audio signals from 20 Hz to 10 kHz with a sensitivity that's large enough to record a LIBS impact up to 5 m on a rock. Currently, the researchers are looking at the commercial Knowles electret microphone, which was considered for the Mars Polar Lander and Phoenix.
While there are plans to incorporate the microphone, though, it will only be once the Mars 2020 rover is launched that we find out whether or not the microphone works and whether it will give us a better look at the Red Planet.