We often read in the literature that the speed of sound depends on the speed of the observer and that of the receiver and you see formulae using c+v and c-v. I feel this is wrong because the velocity of sound depends on the pressure and density for example and similar terms for other matter. In general c^2=p/ρ, where p can be pressure or stress for fluids or stress for solids and ρ is density of the medium. However if the medium is moving along as well, only then the speed of the medium v comes in, but not that of the source or the receiver. What changes with the relative motion of source and receiver is only the frequency. It is interesting to note that Einstein E-mc^2 can be writtn as c^2=E/m and upon dividing both E,m by V the volume containing the two we get c^2=p/ρ, where p now refers to the energy density which has the units of pressure!!