In Rhythmbox 3.4.4 - which is included with Ubuntu 20.04 LTS ("Focal Fossa") - there is the option to "rip" music in MPEG-4 Audio (".m4a") format... Underneath said option, there is a big ol' button that says:
Install additional software required to use this format
But clicking this button doesn't do anything, at all.
I did choose to install Ubuntu Restricted Extras when I did my "clean" install of Ubuntu 20.04 LTS ("Focal Fossa"), but just in case something went wrong, I re-installed it via Terminal.
Someone also suggested that I check that gstreamer1.0-libav and libgstreamer-plugins-good1.0-0 are installed - they both were.
After checking that the above were installed, I restarted my computer and removed / re-installed Rhythmbox - No change (i.e. Rhythmbox still has the same message under MPEG-4 Audio).
This issue affects both my laptop and my tablet.
Any ideas why this is?
Combining Gregory Opera's answer and info from the answer over here, this got me working
I'm not sure which specific package was missing, but this fixed it...
Obviously do this with Rhythmbox and any other media players you have installed closed... Once the installation is complete, you can open them again and the issue should be fixed.
worked my way through Gregory's list, skipped ugly and the first one that wasn't already installed was gstreamer1.0-qt5. Install gstreamer1.0-qt5 and restart Rhythmbox was successful in playing the m4a file