Problem solved!
Here is the manual thing configuration which works.
chromecast:audio:floor [ ipAddress="192.168.1.220" ]
chromecast:audio:sleeping [ ipAddress="192.168.1.219" ]
/* Chromecast Audio Group - this works with the Google Home Mini */
chromecast:audiogroup:House [ ipAddress="192.168.1.220", port=42576]
Furthermore I selected “Chromecast Audio Group” as Default Audio Sink.
When I now use the “say” command like this:
say("The alarm has now been activated ... ")
I get my output on all the chromecast audio devices (all my Google Home devices) which are in the same group as device 1.
Who could update the Chromecast Binding documentation with an example for Chromecast Audio Groups?