The upper frequency limit of digital audio is half the sample rate. For a sample rate of 44100 Hz (44.1 kHz), the absolute limit of frequencies that are supported is 22050 Hz (22.05 kHz). In practice, the limit is a little lower (around 20 kHz). For higher audio frequencies (ultrasonic), you need to use a higher sample rate.
Note that frequencies above 20 kHz are inaudible to humans, and there is no “evidence” (other than unsubstantiated anecdote) that ultrasonic sound has any conscious, subconscious or unconscious influence on humans, unless at extreme intensity which can cause heating effects and ultimately burning.
“Silent subliminals” is a technique that modulates audio with a high frequency carrier wave (usually “amplitude modulation”), in the (unreasonable) blind belief that by some unknown mechanism, the subconscious mind will be able to “hear” the ultrasonic sound, demodulate it, comprehend the “message” and be influenced by that message.
See here for more information about how to create “silent subliminals” with Audacity: https://forum.audacityteam.org/t/silent-subliminals-solved/20100/1