  We determine the velocity of sound in the blackbody gas of photons and in the
gas of relic photons. Derivation is based on the thermodynamic theory of the
photon gas and the Einstein relation between energy and mass. The spectral form
for the n-dimensional blackbody is derived. The 1D, 2D and 3D blackbody
radiation is specified. It is mentioned the possibility of creation of the Mach
cone in case that the high energy cosmical particles moves with the speed
greater than the velocity of sound in cosmical relic photon sea. Key words:
Sound, elasticity, thermodynamics, blackbody, relic photons, Mach's cone.
