I believe it refers to "Lost Media", as in media that is not currently available or preserved in any medium. If Universal were to turn off the Popeye's ride speakers permanently, there would be no way to listen to the BGM loop again.
It would be "Found Media" if Universal were to release the loop on Spotify or Youtube or something, and people were able to download and host it on their own end.
Lots of lost media when it comes to theme parks, mostly just due to licensing and the physical nature of themed attractions. A movie can be hosted on a server, kept preserved on film, or be left on some person's USB stick somewhere and be effectively the exact same experience no matter what. A theme park ride is effectively impossible to fully preserve outside of incredibly high effort Virtual Reality. Luckily, queue and ride POV's, OST reuploads, High Quality releases of ride video like has happened for Back to the Future and Amazing Adventures of Spider-Man definitely fill that void.
Hope we end up seeing the Popeye's BGM released at some point! It's a very niche interest, but it's a really important part of Universal history that shouldn't be forgotten!