Go in to the surround settings, and switch them from “Ambient” to “Full” for music. Then they’ll be a normal set of stereo speakers, and not just a support for the sound bar.
Have you performed Trueplay tuning, adjusted the EQ settings, and have Loudness turned ON in the Sonos app?
In addition to what @Airgetlam posted above, use the Spotify app on your phone rather than the Spotify app through a media device connected to your TV. The “Ambient” and “Full” settings only affect the surround audio in music mode and not TV mode.
Also, be sure the Audio Quality settings for streaming in your Spotify account are set to Very High.
Thanku Airgetlam & GuitarSuperstar for the advice. I shall have a play around with the settings as you suggest. I really appreciate the help!