Alrighty then!
Thank you both for the assist.
Ubunutu Mate 20.04
Yes, Rhythmbox is set as the Preferred Application for "Multimedia Player".
Yep. The last player gets the focus+controls on the sound indicator.
But it was not working for Rhythmbox.
If I run VLC, it gets the focus+controls. If I then open an MP3 in Firefox, it returns to Firefox (last one used).
I removed/purged and reinstalled Rhythmbox. Now it works again. It now gets the focus+controls on the sound indicator.
One odd item, Rhythmbox installs two items that can be selected...
rhythmbox.desktop
rhythmbox-device.desktop
I changed the X-GNOME-FullName to "Rhythmbox Music Player (Device)" to differentiate it. If I select if for the default or preferred, it does not work as expected.
If I use rhythmbox-device, it opens Rhythmbox, but does not play the selected MP3.
I also found the d-conf settings... (these two do not exist on my system: org.freedesktop.MediaPlayer or org.mpris.MediaPlayer2)
.org.canonical.indicator.sound.interested-media-players (these show on the sound indicator).
['firefox.desktop', 'vlc.desktop', 'rhythmbox.desktop']
com.canonical.indicator.sound.preferred-media-players
[rhythmbox.desktop']
Again, thanks for the assist.
The fix was to remove+purge and reinstalled Rhythmbox.
Peace.
--RayJ