The speed of sound is a key parameter for characterizing equilibrium states. However, sound waves change their properties when propagating through rapidly evolving anisotropic media, such as the quark-gluon plasma created in heavy-ion collisions. This paper uses $\mathcal{N}=4$ super-Yang-Mills theory to numerically study the time evolution of the speed and attenuation of sound modes along with the relaxation time in a plasma undergoing Bjorken expansion from various initial states in a quasistatic approximation. The longitudinal Bjorken expansion breaks the isotropy, resulting in two distinct sound speeds that range from just below the conformal value to the speed of light. An anisotropic hydrodynamic description is constructed and its applicability is discussed. Implications for the analysis of heavy ion data are considered.

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• Dissipative dynamics
• Finite temperature field theory
• Nonequilibrium statistical mechanics
• Quark-gluon plasma
• Relativistic heavy-ion collisions
• Strong interaction
• Supersymmetric models
• Thermal properties
• Transport in heavy-ion collisions
• Transport phenomena
• Field & string theory models & techniques
• Hydrodynamic models
• Many-body techniques
• Nuclear many-body theory