I don't imagine it will be a straightforward thing to set in...

I don't imagine it will be a straightforward thing to set in ultrasonic range. As I understood it the speaker is able to switch states at a particular ultrasonic frequency, so could operate at that frequency as an oscillator generating ultrasonic sound. That can be used, e.g. the sensor for the blind that was facilitated with the provision of MEMS for research. I believe the gesture sensing capability is based on emission of ultrasonics and listening through a built in microphone - echo-location effectively. But I don't expect it can play sound at a whole range of ultrasonic frequencies unless these were harmonics of the fundamental frequency.