When the source of sound and observer do not move relatively, but the media moves along the line in which the sound travels, the frequency of sound heard by the observer is different from actual. Explain why. 

When the media moves along the line in which the sound travels, then the velocity of sound is, 
                       v = V±u_m 

where,
v is the velocity of sound in still media,
u_m is the velocity of media.
Due to change of velocity of sound, the apparent frequency of sound is different from actual frequency. Hence the frequency of the sound observed is different from the actual frequency.