Based on your other post, it sounds like you’re using cast-web-api with SmartThings, right?
I only briefly played with cast-web-api to improve compatibility with our Album Art Tile, so I’m not super familiar with it, but a quick search seems to indicate some level of support for speaker groups / audio groups.
Do you have your speakers grouped in Google Home? And do those show up as potential devices when you do the discovery in cast-web-api? If I remember correctly, the speaker groups showed up for me with Hubitat’s Google speaker integration and I thought it did the same in cast-web-api as well.