There is no Galaxy app, I’m quite surprised it works at all. Can’t see Sonos producing a specific change or app to do so. After all they don’t seem to be interested in the Apple Watch equivalent, which is arguably a bigger market.
I asked the same question here.
I think the issue is that the music app on the watch is accessing music apps on the phone for play/pause and stuff, but for volume it skips the app and connects directly to the phone’s Media control.
I can’t decide if it’s more of an issue with the Sonos app not respecting the phone’s Media volume control, or with the Samsung phone or watch not connecting the S2 app’s volume control to the watch’s music app when Sonos is selected.
I’m thinking that Samsung figured “there’s no way a media app would be stupid enough to bypass the MEDIA volume control of the phone. We can just use that and make our application simpler.”