"The lower sound speed on Mars does not lower the pitch of the voice,"  "It makes the speaker seem slightly larger, but still in pitch. In fact, the atmosphere of Mars would raise the pitch of the speaker's voice slightly, because of a density effect."

Microphone missions
Other groups have produced simulations of extraterrestrial sounds, based on their own assumptions about atmospheric effects. The nonprofit Planetary Society actually helped set up experiments to record and send back sounds from the Martian surface — but one mission that carried the Mars Microphone failed (Mars Polar Lander, in 1999), while a French mission that was due to carry another microphone was canceled (Netlander). NASA's Phoenix Mars Lander had a small microphone on its Mars Descent Imager, but itproduced no data during that 2008 mission.

Leighton told me he didn't think the Planetary Society's simulated sounds were quite right, and he sent along a sampling of his own simulations. The differences between the sounds are actually subtler than I expected them to be, except for the Titanian waterfall, which actually sounds pretty alien. See what you think after listening to these sound clips: