Relative velocity of sound Relative velocity of sound.
As I know that speed of sound in medium is property of medium. And independent of source motion but depend on motion of audience or observer and motion of medium. But it is not clear to me. Please explain it.
 A: Have you heard of doppler's effect? Read this  http://en.wikipedia.org/wiki/Doppler_effect 
A: Velocity of sound depends on the velocity of the medium, because in the frame of the medium, the velocity of sound is constant. 
It is constant because the velocity can be derived using Newton's laws, and Newton's laws don't change in any inertial frame(constant velocity frame like the frame of the medium). So if you are in the frame of the medium, the velocity of sound will be the same in that frame as what it was in the ground frame.
For eg. If a wind of $400 m/s$ is blowing eastward, and the speed of sound is $300m/s$, the sound waves travelling eastward will have velocity of $700m/s$ eastward, sound waves travelling westward will have a velocity of $100m/s$ eastward, and sound waves travelling northward will have a velocity of $\sqrt{300^2+400^2}=500m/s$ at an angle of $37^o$ to east.
Now if you are in the frame of the observer, the velocity of the medium in that frame will change, hence the velocity of sound will change.
Hope it is clear to you now!
