Press on the black encoder on the left side of the display to enter the "Settings" menu of the M. In this menu you can also adjust some other useful parameters beside the "Master Volume" parameter.
With the left encoder under the display select the menu item "Master Volume".
Now you can adjust the value of this item with the second left encoder under the display.
IMPORTANT: Afterwards the value needs to be stored permanently with the button "Store" on the right side of the display.