Why Not a Sound Postulate?

What, if anything, would be wrong with replacing the light postulate in Einstein’s 1905 formulation of special relativity with a ‘sound postulate’, stating that the speed of sound is independent of the speed of the source? After reviewing the historical reasons underlying the particular focus on light in the special theory, we consider the circumstances under which such a theory of ‘sonic relativity’ would be justified on empirical grounds. We then consider the philosophical upshots of ‘sonic relativity’ for four contemporary areas of investigation in the philosophy of spacetime: (i) global versus subsystem symmetries, (ii) dynamical versus geometrical approaches to spacetime, (iii) the possibility of a preferred frame in theories of quantum gravity, and (iv) spacetime functionalism.