Like I wrote in comment in https://bugs.launchpad.net/ubuntu/+source/rhythmbox/+bug/1283269, after reinstalling Rhythmbox, Rhythmbox plugins and packages related to Sound Indicator and notifications, everything works as expected. I can't tell, if earlier I had the MPRIS plugin enabled, or not - now is enabled.
Like I wrote in comment in https:/ /bugs.launchpad .net/ubuntu/ +source/ rhythmbox/ +bug/1283269, after reinstalling Rhythmbox, Rhythmbox plugins and packages related to Sound Indicator and notifications, everything works as expected. I can't tell, if earlier I had the MPRIS plugin enabled, or not - now is enabled.