When mixing in 5.1 surround, be aware of what's happening in the downmix.
Downmixing means combining the five main audio channels--front left, center, right, and rear or surround left and right--in a certain way to create a two-channel version. In some downmixers, the low frequency effects channel is added in as well.
There are different ways to create a downmix, depending on its purpose.
In one application, a stereo downmix can be created in a consumer decoder box from the incoming five (or 5.1) surround channels, should the consumer wish to listen to a stereo output. In this case, the left front and left rear are summed with the center to create the left output, and the right front and right rear are summed with the center channel to create the right output. The rear and center channels are usually mixed in at a lower level than the front channels.
In another application, broadcasters can create an encoded (matrix or otherwise) two-channel downmix from a surround mix. Networks and many local stations use this type of encoder for their NTSC channels.
This two-channel feed can pass through existing two-channel transmission and distribution paths to the consumer. At the consumer end, a decoder box converts the two-channel signal back into something resembling the original 5.1 channel surround mix.
For consumers without a decoder, this encoded two-channel downmix is simply the audio that comes with the pictures on the TV set, either in stereo or mono, depending on the TV.
We can see, with just these two examples, that to determine downmix compatibility you would need to monitor a simple summed downmix (first application), an encoded downmix in both stereo and mono, and a downmix that has been decoded back to surround (second application).
