Michael Shermer played Stairway to Heaven backwards and it mostly sounds like gibberish. But when he displays the 'translation' as text and plays it again, the words seem pretty clear.
In 1985 two young men attempted suicide, and the family sued the British rock band Judas Priest, accusing them of hiding subliminal messages in the music that said "Do It!" when listened to backwards.
From memory, part of the trial was the defence taking a song, suggesting what words the court would hear in it played backwards, and then playing it backwards. e.g. this clip https://youtu.be/uyIsu93zAoM?t=1123 and showing that you hear the suggested words out of the noise.
The trial was thrown out, after deciding there were words and they were subliminal, but were not deliberate. The whole situation included: "In a pre-trial motion, the judge ruled that subliminal messages were incapable of being protected speech under the First Amendment to the United States Constitution, since they were by definition not noticeable and thus could not form part of a dialogue."
https://www.youtube.com/watch?v=wYG2oZXSvdE