No1DaBeats correctly reported that the AI knob can control the main mix level, you do not need to have your mouse over the control however, the AI controller has a dedicated Vol mode which allows control of the main mix irrespective of the mouse. 
No1DaBeats correctly reported that the AI knob can control the main mix level, you do not need to have your mouse over the control however, the AI controller has a dedicated Vol mode which allows control of the main mix irrespective of the mouse.