I have a 2.1 setup: two front speakers and a sub, fed by a Yamaha receiver (RX-v667), which is fed by my Win7 x64 computer over HDMI (AMD 6950), with MPC-HD as the source player. My receiver's current mode is set for 7ch Stereo. (There's no mode for 2.0/2.1 stereo)
When playing movies, if the movie has more than one audio track -- like a 2.0 track and 5.1 track -- the overall sound is much louder with the 2.0 track than the 5.1 track (which I switch between using the LAV Splitter Source filter).
From what I can gather, this supposedly has to do with the 2.0 track all being fed to my two speakers, whereas the 5.1 track is being split into six separate channels, and then only feeding my two speakers (and sub) their respective slices of the track; all other surround channel slices are lost without having an actual 5.1 speaker setup to play them.
I've read that even though I have a 2.1 setup, it's still preferable to use the surround audio track, for some reason. I'm not sure why. And if so, I'm guessing I haven't configured it correctly, perhaps needing to set somewhere to have all 5.1 channels routed into the two speakers and sub.
(... If so, I'm not sure where that'd be preferable to implement -- having the receiver somehow combine them, having Windows combine them, or having MPC-HD (and/or filters, like my LAV filters) combine them. I can split this into another question, if need be)