To support Lee's buffet mechanism model [B. H. K. Lee, “Self-sustained shock oscillations on airfoils at transonic speeds,” Prog. Aerosp. Sci. 37, 147–196 (2001)10.1016/S0376-0421(01)00003-3], the sound wave propagation in the flow field outside the separation of a transonic buffet flow at a Mach number M∞ = 0.73 and an angle of attack α = 3.5° over a DRA 2303 supercritical airfoil is determined using high-speed particle-image velocimetry. Furthermore, the shock wave is influenced by an artificial sound source which evidently changes the shock oscillation properties. The dominant buffet mechanism is shown to be a feedback loop between the shock position and the noise generation at the trailing edge of the airfoil. The sound wave propagation speed is detected by correlating the surface pressure signals and the velocity fluctuations in the flow field. The quantitative results for the natural and the artificial sound source convincingly coincide and are in good agreement with a reformulated version of Lee's buffet model.