https://spectrum.ieee.org/semiconductors/devices/how-we-reve...
"Computer science researchers have explored the physics of IMD. In the DolphinAttack paper, we used ultrasonic signals to trick a smartphone’s voice-recognition assistant. Because of nonlinearity in the smartphone’s microphone, the ultrasound produced by-products at audible frequencies inside the circuitry of the microphone. Thus, the IMD signal remains inaudible to humans, but the smartphone hears voices. In an early 2017 paper, Nirupam Roy, Haitham Hassanieh, and Romit Roy Choudhury at the University of Illinois at Urbana-Champaign described their BackDoor system [PDF] for using ultrasound and IMD to jam spy microphones, watermark music played at live concerts, and otherwise create “shadow” sounds."
Your Siri is talking to the voices in her head... Conspiracy theory wise i'd think (ie. hope that "at least some of my tax dollars at work") that NSA uses such tools too, and this is why everybody is so mum about it.