Audio fingerprinting works because small differences in browser versions, operating systems, audio libraries, and hardware can produce slightly different results from the same generated signal. It is not necessarily enough to uniquely identify a device by itself, but it becomes much more useful when combined with canvas, WebGL, hardware, timing, and interaction data.

The gaining happens after retrieval of the results it seems: so it’s just silent for the end user. And perhaps they do both fingerprinting, and ensure audio devices remain awake.