Music MiniPlayer has an array of ways to control volume, from keyboard shortcuts to dragging and scrolling. Refer to the list below to find your preferred way of adjusting volume in Music MiniPlayer.
Using keyboard shorcuts: press ⌘↑ or ⌘↓
Place your cursor arrow over the Volume Slider Knob and drag left or right
Place your cursor arrow over the Volume Slider and scroll your vertically or horizontally
Click the leading speaker icon to mute the volume. Click the trailing speaker icon to set the max volume.