It is a premise of the Galilean principle of relativity that every reference frame behaves mechanically like an enclosed compartment at rest.  As a consequence of this premise it is presumed to be mechanically impossible to discern the motion of any reference frame by observing experiments conducted within that reference frame.  Material objects in flight within an enclosed compartment will manifest a particular velocity that arises from momentum transfer through physical contact with the compartment walls.  Objects in flight outside of the compartment will exhibit essentially the same behavior via contact with the external physical structure of the moving compartment.    However, a sound wave in flight through an enclosed compartment where the air has no wind currents in it will manifest one particular velocity while a sound wave propagating through the still air outside the compartment will manifest some other velocity — in a moving enclosed compartment the contained air’s velocity is the same as the compartment’s velocity and would add to or subtract from the sound wave’s propagation velocity.   There is then a difference in the mechanical behaviors of material objects and sound waves when they are moving through any particular medium based on whether that medium is within or outside of a moving enclosed compartment.  Under certain conditions an observer in a stationary or moving reference frame may not have to apply the principle of addition of velocities from the Galilean or Lorentz transformation equations to the propagating sound wave.  Not every reference frame is an enclosed compartment.