Hi,
To set the volume, you can look at the setVolume() method for the AudioOutput class.
However I’ve tested myself on Processing 3.5.4 and it seems that this method is not supported :
So you can use the setGain() method in the mouseMoved() function that accepts decibels :
out.setGain(((float)mouseY / height) * 10);
However when printing the gain to the console, it stays limited at 6.0206 dB… Maybe someone have the solution 
(mouseY / height return a number between 0 and 1 and since you want to have a float division, you cast the first number before)