Right now, the SmartThings official features don’t give you granularity. You can turn the speaker on and off, and you may be able to have a play voice notification depending on the speaker capabilities, but you can’t select a specific station or playlist.
Option 1: use a harmony activity
If your multiroom system does work with harmony, you can set up a harmony activity for a particular favorite station and then you can have that harmony activity triggered from SmartThings. It may not be everything you want, but it is fairly easy to do. Sonos is a good example.
Option 2: use custom code for a specific configuration
There are some individual community members who have done some projects that can do some granularity for some specific devices. And if that sounds like a lot of qualifiers, it is. There’s no one smart app that can handle granularity in general.
And in many cases these will require quite a bit of technical expertise as you may need to also set up a separate “man in the middle” server to work with it.
@ule has some code that can select stations from a specific service, “radio tunes.”
And @infofiend has done a lot of work on Sonos integration down to the playlist level.
You will find both of these and more on the quick browse lists in the community – created wiki under project reports for A/V projects.
So…not simple, not easy, but may be doable for some specific configurations.