You may select which tracks are played, and their relative volume, by using the Audio Mixer menu option from within the Viewer Window.
To control the volume of a track use the slider for the track you would like to adjust. To silence a track entirely, click on the Mute button.
When you exit the Viewer Window, the instrument and voice levels you chose will be retained for the next time you open the piece (even if you sign out, close the app, or remove the song from your computer). You can change the instrument mix both during playback and while stopped.