I had not looked at the data sheet before my first reply. After looking at the data sheet it looks like the I2C control is just to select 4 preset gain values. (12, 20, 26 or 32 db) This would not be enough to use for a volume control. The only analogue inputs I can see are the audio inputs so you could just use a normal dual log potentiometer for volume control. Trying to control the gain by adjusting the supply voltage is a bad idea. It would result in clipping which would cause distortion.
Les.