I don’t personally call it backmasking when it’s just reversed audio that sounds like gibberish forward but with a backward message when reversed, nevertheless Slashfilm has an article about the title cards at the end of each episode of It’s Always Sunny in Philadelphia which have a new secret message for each season.
The production company logos that air at the end of TV shows offer creators a chance to put their own creative spin on things one last time before their time slot is up, and comedy shows tend to make it fun. “It’s Always Sunny in Philadelphia” not only had fun with their production company logo, but they’ve also changed the audio that plays each season. There’s just one major twist on things: they backmask all of the audio!
They’ve got the text of the reversed audio for all 16 seasons.