In a swift river where water flows faster than sound, a shout from the bank will be carried downstream but never return. The boundary where the flow speed matches the speed of sound is the sonic horizon. Below it, silence reigns. That very same boundary is created by infalling gas around a black hole. This was described mathematically by Schwarzschild. At the threshold of the sound barrier, the vacuum begins to produce quanta of sound — phonons. This process is the acoustic twin of Hawking radiation. An unexpected twist: spacetime curvature acts like a loudspeaker. It shifts frequencies upward, creating an analog of the blueshift of light, and makes the sound hundreds of times 'louder' than the temperature alone would allow. So in the noise of infalling gas, gravity intertwines with quantum fields and opens a window onto the mysteries of singularities and quantum thermodynamics.