The most popular method for creating “subliminal messaging” is with “amplitude modulation”. This is similar to AM radio, except that it is done with high frequency sound waves rather than radio waves, and is at a much lower frequency than radio.
There is a “Nyquist plug-in” available here for this: https://forum.audacityteam.org/t/silent-subliminals-solved/20100/3
Installation instructions here: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_linux.html#nyquist_install
Note that MP3 encoding is likely to strip out high frequencies as MP3 is designed to remove information that is inaudible (so as to reduce the file size while preserving the audible information).
“supraliminal” means “existing above the threshold of consciousness” / “adequate to evoke a response or induce a sensation”.
A normal audio recording played at normal listening volume is “supraliminal” because you can hear it.
“subliminal” means “inadequate to produce a sensation or a perception” / “existing or functioning below the threshold of consciousness”. In other words, you can’t hear it.