To get the details go to www.mitsubishi-tv.com
The 2011 models have model numbers that end in the following last three digits.
C11 - entery model (good)
640 - basic model (good)
740 - step up model (better)
840 - Diamond Series model (best)
The 2012 models have model number that end in the following last three digits.
C12 - entery model - not 3D (good)
642 - basic 3D model (good)
742 - step up model (better)
842 - Diamond Sereis model (best)
As for the sound system - that is a difference or step up between the 740/742 series and the 840/842 series. Not a difference between 2011 and 2012.
All have good to exellent pictures - the step ups mostly deal with other feature upgrades. Some of these feature upgrades may be important to some and not others. Such as the higher end audio or additional color controls of the 840/842 series or the internet features of the 740/742 and 840/842 series. Sometimes you need to purchase some extra features you do not want in order to get the extra features you do want - however this is the way of life in almost everything.