There are many techniques used for embedding subliminal messages in audio files. In order to “decode” the message you need to know what method has been used to encode it. “Amplitude modulation” is one such technique and there is information about decoding that type of “silent subliminal” in this topic: https://forum.audacityteam.org/t/silent-subliminals-solved/20100/1
Also probably worth mentioning that there is little scientific evidence that subliminal messages “work”, and no credible scientific evidence supporting “AM”, “FM” or “frequency shift” methods of encoding subliminal messages.