You can turn it down quite a bit with the volume slider. 

Does the Surrounds on/off toggle in Settings/System/(room name)/Surround Audio not suffice?

I wasn’t sure here, if the OP really meant to say they wanted the option to switch off the main HT Soundbar (player) instead, as they do go onto mention being able to TruePlay Tune their rear surrounds only.

I’ve seen that request put here before in the community, where a user wants music playback through two wider and separate surround speakers only and their request was often answered by a suggestion of buying the new Sonos Amp instead, which would not only provide them with wider stereo separation for TV and music audio, but the speakers would then be out in front for the ‘sweet spot’.  ...(and I guess it could also be behind too, if using two Sonos Amps).
 

The option, as @ratty has mentioned in his post, has been there in the App to toggle off/on the rear surrounds, at least for as long as I can remember, so I’ve never seen anyone previously ask for that option in recent times, but I can see a case where some might want to mute the main HT device and just have the music audio play through their rear surrounds for wider stereo separation, though it’s still not really ideal to have the music soundstage located behind the listener.
 

If affordable and practical, it’s sometimes perhaps far better to add two further Sonos speakers out in front as a separate Sonos Room for stereo music audio, which I presume, is what Sonos would like us all to do anyway, from a sales perspective. I would certainly do that with my Sonos Beam, if I could ever play (in lip-sync) the front left and right TV audio channels through both those “out in front” speakers too.