Downmixing is folding a multichannel mix into fewer channels — 5.1 to stereo, or stereo to mono — so surround content still plays correctly on a smaller system. It is not a simple sum: standardized coefficients control how much of the centre and surround channels go into the left and right, and the LFE is usually attenuated or dropped. The classic stereo downmixes are Lo/Ro (a plain stereo fold) and Lt/Rt (matrix-encoded so a Pro Logic decoder can recover surround). Done carelessly, downmixing makes dialogue drop too low or phase cancellation hollow out the sound, which is why broadcast and streaming specify the coefficients and check the result.
