The 40NX713's audio performance is serviceable but thin, and stereo imaging is cramped, if not entirely monophonic. This is not surprising for such a slim TV design.
Providing modest welly is a diminutive S-Force digital amplifier with a claimed 26w total output (this is derived from a 2 x 8w stereo output, plus a 10w subwoofer). Given that there's no real bass coming from the set, the term 'subwoofer' is a bit of a misnomer: it's more of a mid-bass plumper.
For those that want to experiment, there is a selection of DSP modes: Cinema, Music, Live, Sports and Game.
Ultimately, it's well worth augmenting the set with a separate sound solution when circumstances allow.
When all's said and done, the 40NX713 is an outstanding flatscreen TV. Given that it's widely available for less than a grand, it can be considered rather good value for buyers seeking an above average 2D Full HD performer.
In many ways, the 3D element appears to be an afterthought. Having to add an external sync transmitter seems awkward at best. With the best 3D content available, the set offers tangible depth, but the bugbear of crosstalk is always there if you look for it. You should regard the 3D component as a bit of fun, no more or less.
The only significant negative in terms of picture performance is the uneven backlight, but then this has to be traded against the ultra-thin cosmetics. If it bothers you, perhaps you shouldn't be looking at edge-lit LED as a screen technology.
Ultimately, we would rate the KDL-40NX713 as a great Full HD picture performer.