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.