Thinking that I must be missing something, I took some time last night and played around with this on my new Pixel 8 (Android) phone, thinking that maybe my old Pixel 2 was to blame for issues with casting SiriusXM to the Wiim Pro's (no mini's). After my fresh revisit – here's what I noticed -
Casting through chromecast from the XM app to just one 1 Wiim Pro works flawlessly and is easy to control.
Casting to more than one Wiim Pro is where the problems start -
-If you cast inside the XM app to just one Wiim pro and then go to the Android Wiim App, you can link multiple Wiim Pro's to the one that is being casted to, however, sound doesn't come through to the other Wiim Pro's, just the Wiim pro that you are casting to on the XM app. So that route is a no go.
-Then, thinking that I should be using the Android Home App to control the chromecasting, I found that I could successfully cast to multiple Wiim Pro's and control the volume on each Wiim Pro inside the Android Home App – however, at that point the XM app becomes nonresponsive. Trying to change the channel or stop the casting in the XM app doesn't work. Also, trying to stop the casting in the Android Home App also doesn't work either at that point. I found that I had to go back to the Wiim App to stop the Wiim Pros from playing. Then at that point, even after exiting out of the Android Home App, the Wiim App, I still had to reboot the phone to exit out of XM. Further, setting up a speaker group inside of Android Home App doesn't seem to help with this issue.
This is similar to the experience trying to cast to more than one Wiim Pro with my Pixel 2 phone – it would generally make the phone unstable and cause a reboot. I learned from several attempts with that phone to just not "go there".
I could live with using the Android home app to cast to multiple Wiim pro's if that worked, however, having to restart everything to change a channel and dealing with 3 apps isn't exactly easy to explain to other household members.
If there is another way that someone can think of to cast XM to multiple Wiim Pro's – I'm all ears. I can think of buying a Sonos port and connecting it to one Wiim via RCA and then using that to send link to other Wiim's (the reason I say Sonos is that their App works well with XM). But that setup isn't exactly user friendly either and then makes me start to rethink about how to handle whole house audio in 11 zones.
I wish that the XM app worked as well as the iHeart App does with the Wiim's.
Maybe these issues don't exist with airplay and an old iphone would be the solution? Does anyone have any experience with XM, airplay and multiple Wiim's?
I guess where I'm at is that if these problems aren't something that can be resolved by Wiim, then they should clarify in the FAQ for SiriusXM that chromecasting to multiple Wiim's is problematic. That would be a start, then if they can't get the XM app to integrate and play nice with the Wiim's, then for the sake of transparency, they should remove it from the Roadmap so others don't have the expectation that its coming.
I realize that not everything is possible, but the impression was given at the beginning of this year (or earlier) that XM integration was on the way and almost a year later it's not happened. Overall from a multizone, non-high fidelity user standpoint, there are many great things about the Wiim products to which Wiim should be commended for ... but this is the one item I would say is missing.
Greatly appreciate any efforts in this area if someone from Wiim development is listening!
I use Sonos all the time to listen to XM (great implementation … better than Alexa’s), but would much rather it be on my WiiMs if they would build the integration like they did with Pandora (XM owns Pandora). I’m slowly replacing my Sonos system with WiiMs, but really need both XM and Plex within WiiM to pretty much complete it. Plex on Sonos works great too for my needs … so, it’s possible WiiM! (Granted … it’s no Plexamp integration which would be even better) 
