At the upper right is an icon that looks like two switches on top of each other. This gives you the "control center" which includes sound control.
I can choose whether the mini or the monitor do the sound. It is currently selected to use the mini. With the mini selected I get a volume slider, but not if I select the monitor.
If I had a proper apple keyboard, it would have keys to raise/lower, or mute the sound.
F10 is the mute F11 would lower volume F12 would raise volume
I can control the monitor volume from the OSD menu in the monitor. I press the joystick button to get the menu, move up/down to find sound and press right to select it. I can then select Volume and use the joystick to change the level.
I was worried that when I began using an HDMI cable for the monitor I would get unwanted sound, but this is not an issue.