Record subliminal vocal messages. Musical lyrics are filled with metaphors and other writing devices that suggest something without saying it directly. A lot of artists have also used the echo effect and other mixing procedures to place an underlying or subliminal message under the main bridge or chorus. After you record the vocals, lightly mix the vocals into the track at a low volume and with some type of effect such as an echo.
Add sounds that suggest mood. Music is full of different sounds, and they all produce different moods. For example, a violin often produces sweet but sad sounds. This is great to add onto a musical track that represents death, reflection or sorrow. Use musical instruments to your advantage or to contradict what you are saying (if that is the purpose). Mix the sound deep into the background of the song. Think of the songs as textures, with the subliminal message deep under the root of the track.
Add other effects and sounds to your recording. For example, a song protesting police brutality could simply come out and say "Cops are bad; they take advantage of the weak," or the song could use subliminal messages to state the same belief. Maybe add dissonant sounds to the recording while you sing praises about the cops. Always mix to contradict or add underlying support to your song for maximum subliminal effect.