Thanks for taking your time to report this issue and help making Ubuntu better.
The gstreamer1.0-fluendo-mp3 package was indeed removed during the Cosmic development cycle, see https://launchpad.net/ubuntu/+source/gst-fluendo-mp3/+publishinghistory or https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=899838 for details.
So it looks like the remaining part is updating ubuntu-restricted-addons to recommend gstreamer1.0-plugins-good instead.
Thanks for taking your time to report this issue and help making Ubuntu better.
The gstreamer1. 0-fluendo- mp3 package was indeed removed during the Cosmic development cycle, see https:/ /launchpad. net/ubuntu/ +source/ gst-fluendo- mp3/+publishing history or https:/ /bugs.debian. org/cgi- bin/bugreport. cgi?bug= 899838 for details.
So it looks like the remaining part is updating ubuntu- restricted- addons to recommend gstreamer1. 0-plugins- good instead.