Proof of concept - converting block diagram in US patent 5,159,703 to functional schematic
This circuit reverses the spectrum of the speech (high pitch becomes low and vice versa). Additionally everything is shifted to nearly 15 kHz. So something high pitched hissy whistling is available and something may be recognizable, too by people who are young enough. I have over 60 years old ears that are damaged by explosions and shooting - needs a jet to make something hearable to me even at 10 kHz.

Balanced modulator is a switch that selects the signal "as is" or the polarity inverted. Carrier oscillator 34 swaps the selection for every half cycle. An analog multiplier also is possible, if it works at 455 kHz.

The filter 35 is a bandpass  one that saves  maybe 20 kHz around 455 kHz. Its bandwidth is not critical - can be as well 20 or 50 kHz.

The mixer and the heterodyne oscillator are of MW radio technology, but a ring modulator or analog multiplier go as well. The bandpass filter 38 should remove everything below 14,5 kHz.

It's possible to test the idea in software simulation. I have done this kind of tests in Vissol's Vissim. It's an old product, at least 20 years. It accepts and produces WAV files if audio is needed. That company has been sold maybe 1 years ago but their product Vissim can still be available as demo in download sites or in the Wayback machine.

Actually even no speech is needed for some messages. Random high pitched noise is enough. Supermarkets and other public sites have a problem: Unwanted, non-buying people accumulate inside or at the main door and frighten the valuable customers. The owner assembles a high pitched whistler that makes the loiterers to feel the place uncomfort and go to elsewhere.