Thanks! I think I'll cut my time-losses and swap my two Chromecast Audio
dongles out for Bluetooth receivers, or a new pair of Bluetooth
speakers. Great solution and likely better supported.
Yes, I believe there are two issues, and the newest, larger one sucked
the air out of my hope for Google's solution for the preceding one this
thread is about.
There seem to be two problems currently - one that has existed for at
least a month, as documented in the early posts in this thread, and a
new problem that began within a few days ago. Are they related? I don't
know.