Hi everyone!
I was also casting music almost every day with Poweramp to my Nest Mini speakers (I've got 4 of them), and this Monday I immediately noticed that it didn't work... I thought at first that some update to Google Play Services or Nest Mini firmware must have caused it (Poweramp Android app hasn't changed for some months), but after reading here that other music apps that cast local files still work then I guess it must be something about Poweramp's code that got broken due to a server-side change by Google.
Volume control of the speaker still works through Poweramp, regardless of no "pling" sound and being unable to play music.