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It's not something I've looked into properly. I saw some app recently called Plex Together that may do what you want or you can do it by choosing your hardware (ie. chromecasts, sonos or bluesound) and using the grouping at that end.
Otherwise, there may be a convoluted solution involving installing a squeezebox server and using Kodi rather than Plex as the front-end and using the unofficial Plex and Squeezeplayer add-ons in Kodi. (I played about with this a while ago and got bored of trying to make it work nicely).
I don't think Plex is great at multiroom audio from its interface.
In terms of the chromecast audios I think you can set them up as one listening group so you can cast to multiple ones (only a solution for chromecast though).