rhythmbox-client --print-playing tells me the name of a song, regardless of whether it is actually playing. I simply need to know whether or not sound is currently being generated by rhythmbox (so that I know whether or not I want to pause it and later unpause it).
One ugly candidate answer:
I think rhythmbox really may be devoid of this basic interface.
But when I run
which I gather queries the mixer about what is feeding, it still lists rhythmbox regardless of whether it's playing. However, it has a "state" row in the output, which is "RUNNING" or "CORKED" depending on whether the music is paused.