For music, under Surround Audio in the Sonos app, you can set Music Playback to Full and you can increase the Music Level setting.
For TV and movies in stereo, the best you can do is increase the Surround Audio TV Level setting.
FYI… this is from Sonos:
If the source you’re playing isn’t in 5.1 already, your Sonos home theater speakers will perform an audio up-mix of the stereo signal to create a simulated surround sound. This means you’ll still hear most audio out of the front channels, but the rear left and right speakers will play what we determine to be ambient audio. This is most obvious in action scenes and with movies that have intense soundtracks.