When viewing or playing a score in the mobile app, the gear icon "⛭" on the top right takes you to the Part Mixer.
The horizontal sliders allow you to independently adjust the volume of the different instruments in a score.
The eye icon allows you to selectively hide and show instruments from the score, enabling you to view only one part at a time if you wish to do so.
The "Solo" button allows you to enable or mute individual instruments in the score when using playback.
When done, close or swipe the menu down to go back to the score.
Keywords: mixer, parts.