Implement "Invert Selection" in the Mixer
This will come in handy if you want to hear just one instrument or a selection of instruments and mute the others. Select the instrument(s) you want to hear and then check "Invert Selection". Now all the other instruments are muted, and the one you selected is unmuted. You can even implement "Invert Special..." which would allow you to select by instrument types (strings, brass, percussion, winds, voices) allowing you to isolate groups or choirs.
Comments
This would be useful indeed. Meanwhile, the way to quickly hear individual sections is to just create "parts" for them - a woodwinds part, a brass part, etc. Parts aren't limited to one per instrument; you can easily add a part containing any group of instruments you like. Also, MuseScore 4 will allow you to solo any group of staves simply by selecting them prior to playback.