Audio balance changes randomly in MacBook Pro Retina 15-inch' mid-2012
I've had this problem on multiple MacBooks (from my old Powerbook 15 from 2003 to prior to finding this question on my late 2011 MBP 13").

Apple has acknowledged it as a bug when pressing the volume buttons during heavy CPU usage way back in OS 10.2! The solution was to go back to preferences and set it back where you wanted.