There’s a topic here that discusses implementing “silent subliminals” in Nyquist: https://forum.audacityteam.org/t/silent-subliminals-solved/20100/1
To the best of my knowledge there is no verifiable or peer reviewed scientific evidence to support the efficacy of “silent subliminals” in any form, so I’m not able to comment on that.
The theory behind AM and FM (modulation) are described here:
http://en.wikipedia.org/wiki/Amplitude_modulation
http://en.wikipedia.org/wiki/Frequency_modulation