Follow the instructions here: smartthings
Afterwards, you can use the “Speaker Companion” smartapp to play sounds.
In my case, I installed the RELEASE Generic Media Renderer (DLNA Speakers). Cheap Sonos Alternative (Update V2) smartapp to trigger sounds or to render custom speech.
For playing particular online “stations”, I crafted URLs that follow the castweb URL scheme and saved those URLs presets as buttons. For example:
When that URL is posted to the castweb server (http://my.fakedomain.com:3000), my Google Home speaker group (192.168.1.70:42929) is instructed to start streaming the specified mediaURL (http://167.114.131.90:6100/;stream/1).