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Mars, Venus and Titan have an atmosphere and we can theoretically step on them (and die quickly).

So, as an example, let's say we can survive on these objects without a space suit. I say: “Hi” on any of these objects. Would I hear what I said or not?

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If there's an atmosphere, there's sound. As simple as that. It doesn't matter what the atmosphere is made of. As long as pressure is non-trivial, you will get some sound transmission. –  Florin Andrei Jul 21 at 19:43
    
Ok. So, what happens when there is more or less atmospheric pressure? –  Archa Jul 21 at 19:51
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Put your head under water and you'll see what a denser atmosphere means for hearing. Sound travels faster in water than in air. If you dive and hear a distant torpedo explosion, and then quickly put your ears over the surface, you might hear the same explosion again, because of the differences in sound travel time in less and more dense "atmosphere" (air and water). –  LocalFluff Jul 21 at 19:58
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These comments should really to be posted as 'answers'? Thanks. –  Robert Cartaino Jul 22 at 14:56

3 Answers 3

It depends on the composition of the atmosphere. In a helium (or other light gas) atmosphere, your voice would have a higher pitch. If the gasses are heavier, your voice drops.
The density is also a factor: on Mars, sounds can't be as loud as on Earth. Sound is transmitted as pressure differences, and the largest pressure difference you can get is that between the atmospheric pressure and a vacuum.
The speed of sound also changes with the composition and pressure of the atmosphere. This would change the way you perceive the distance and direction of an object that makes a sound: we normally use the tiny differences in time between a sound reaching our left ear and our right ear. A different speed of sound screws up those time differences.

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Yes, you and everyone around you would hear you. The anatomy of speech might complicate speaking in exotic environments, but we can be heard breathing helium, and from underwater. Sound is vibration of a substance, or medium. The medium a sound is transmitted across will make no difference in whether or not it is transmitted, but will impact how far it audibly travels (i.e. the rate of diffusion of the sound wave).

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Just as an addition to the previous answer:

Some spacecraft that landed on other cellestial objects carried microphones to record what can be heard.

For example this video http://youtu.be/36ffV-CI3Mo contains a sound clip recorded by the Huygens probe during its descent on the moon Titan.

As far as I remember there are similar recordings from Venus, as short a time as some probe can survive on this beautiful hell hole :-)

So yes, there is definitely sound on other planets if they have sufficiently dense athmospheres.

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