iOS/Android
(Mac, see below)
Tap on the chart to reveal the playback control.
The Mixer is the second button from the right at the bottom of the player control.
Mixer Settings (iOS, Android, Mac)
You can change the playback instruments' sound and volume and the desired level of Reverb using the volume sliders.
(Mute instruments completely by sliding the volume to the far left.)
Mac
The mixer is in Player Controls (the right panel)
Use the View menu (to show/hide) > Minimize/Maximize Player Controls, or click the Player Controls icon (top right)
Click instrument name to change instruments, sliders for volume
