Assign a slider or pot to the volume control
Global Functions
Pick any unused one
Set 1st to On
Set 2nd to Volume
Set 3rd to the slider or pot you want to use by sliding or turning it
Tick the checkbox at the end
Return a couple of time to get back to the main screen

For me I used the right side pot, turn it to the left to turn the volume down or all the way to off turn to right to increase to maximum.