(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.
The mixer is on the right panel, Player Controls.
Use the View menu to show/hide: Minimise/Maximise Player Controls, or click the Player Controls icon top right.
Mixer Settings (iOS, Android, Mac)
You can change the playback instruments' sound and volume and the desired level of Reverb.
Mute instruments by sliding the volume to the far left.