Question

why the medium through sound travels must be frictionless ?

Answer 1

The medium should be frictionless so that there is no loss of energy in the propagation of sound through it. ... When a source of sound vibrates, it creates a periodic disturbance in themedium near it. The disturbance thentravels in the medium in form of waves. Sound travels in air in the form of longitudinal waves

Answer 2

You must know that the sound propagation is in form of wave ,such as this generates a periodic disturbance in its medium or near mediums, due to its wave structure.

Now if the medium is not frictionless the travelling of sound may eject more energy than required so their will be energy loss! And it creates disturbance which also can travel into the space anywhere. To minimise this noise or disturbance or energy loss, whichever you can say the medium should be frictionless