technically, if you send 2 channel sound to a system that is set up for 5.1 or 7.1 channel, nothing is actually disabled. There is just no content fed to the unused channels, so they are silent. On the flip side, as I look at it, at least, when I (almost always) expand stereo sound into 5.1 on my system, my receiver "invents" the center (a pretty straightforward thing) and the surrounds (more electronic guesswork is involved) and I will either like what the receiver invented or not. The preamp and amplification channels, as well as the speakers, are all just sitting there waiting to be used, if the thing is turned on. I spent some time making the invented surround work well for me for most music, and like the bigger soundfield. I have zero problems with those who prefer to hear the 2 channel stuff in its native format, or prefer more dramatic surround extraction than I do.
If you are feeding a soundbar or similar system from your TV, and you decide you actually want multichannel, you should be aware that some TVs just will not pass through some multichannel inputs (except for over-the-air). HDMI connections seem to be a particular problem (the TV tells the source device it's only a stereo playback system, then the source device will refuse to even send the multichannel signal to the TV. Not AIUI, an issue with optical or rca digital connections.
But if you are playing back through TV speakers, send the stereo feed and do nothign else, and it will work as you expect, AFAIK (subject to vagaries of different brands of equipment.)
