Create or choose an audio track that you want to add your subliminal message to. Save it to your computer in either .wav or .mp3 format.
Open Audacity and record the message you want to use as a subliminal message.
Highlight the entire track, then go to "Effect" on the menu bar and select "Amplify". Choose a negative value for amplitude. This reduces how loud the sound is. Repeat this process until the exact point you are unable to decipher the words in the track.
Select the entire track, go to "Edit" on the menu bar and select "Cut".
Open the file you want to add the subliminal message to.
Go to "Project" and select "New Track".
Click on the new track, go to "Edit" and select "Paste". This places the subliminal message within the track. Move the track to the exact location that you want it to be played in the sound file.
Save the file by going to "File" and selecting "Save Project" or "Export as Wav" or "Export as MP3".