Today I noticed Rhythmbox not playing some music files in Debian. After a bit of digging around, I noticed the warning “your gstreamer installation is missing a plugin” in the file properties. From there it wasn’t far to realize that MP3 files did not play in Rhythmbox (any more), and that I probably need a (new) plugin (it’s Debian ‘unstable’ after all, so this was probably introduced by a recent update).
sudo apt install gstreamer1.0-fluendo-mp3
and restarting Rhythmbox was all I needed to do.