Question about mixing 6 mono channels down to 2

[rado84]

I'm using SMPlayer with mpv as a backend. I've set it to always mix down channels to 2 bc that's my audio system - 2 speakers and 1 subwoofer. And when the sound is mixed by SMP/mpv, it's perfect. But when I extract it from the mkv and mix it down using Audacity, the result is always mono. Somehow I managed to make it stereo but it sounds awful and there are short distortions that sound even worse. For the lack of a better word it sounds like a... "forced stereo".

So my question is: is there any way I can have 6 mono channels mixed down to 2 stereo channels and retain the same sound quality I can hear in SMPlayer/mpv?