Hope this might help at least some people who have a Chromecast Audio (or Chromecast Gen2) device which has been rebooted since the recent problems. By chance I had found out an old Chromecast Audio device recently which hadn't been used since I'd changed my WiFi setup and so wasn't able to connect to my WiFi - so after trying to connect to my old network I'm assuming it gave up and fell back to a state which is very similar to a factory reset device (i.e. it sets up it's own WiFi access point which the Google Home app connects to in order to configure the WiFi settings etc.). In this state it can't connect to a network so can't download the new firmware.
Today I had an idea - I have a Chromecast Ultra device in an area that had poor WiFi reception and so I had it plugged in via one of the official Google mains adaptors which has an ethernet connection in the plug. I plugged this adaptor into my Chromecast Audio and connected the adaptor to my ethernet network - the Chromecast audio booted up and immediately downloaded the new firmware (shown by a blinking orange light). I gave this time to complete the download and reboot and then returned it to it's normal adaptor. I then followed the normal setup process in Google Home and this completed OK. My Chromecast Audio lives again!!
I imagine the same process will work with a Chromecast Gen 2 device. So if you have reset your Chromecast Gen 2 or Chromecast Audio and have access to a Google mains adaptor with ethernet connection (I would imagine that any of the adaptors with a micro USB connection would work) then you might like to try this to get you up and running before the official Google solution is available.
