Some musicians and audio engineers have used a technique called backmasking to insert hidden lyrics into songs. When a song is played backwards, only then can the new backmasked lyrics be heard.
There is some skill required for this technique that requires coherent lyrics when played both forwards and backwards. Success in this technique is built upon carefully choosing word combinations that sound like other words when played in reverse. For example, the word "one" is heard as the word "now" when played backwards.