impress cannot read transition sounds

I seem to be unable to add my own transition sounds to a presentation. When I try to add an mp3 as a transition sound to a slide, I get the message “The file … is not a valid audio file.” The system sounds in /usr/lib/libreoffice/gallery/sounds can be loaded, but if I copy my file there, it will not load. Previously created presentations are working correctly.
ubuntu 20.04 LO

Can this help?

Media Support