I have the logitech z-560 speakers and the m3d mode doesn't do anything at all. I have tried a few different setups. The first was when I was using AC'97 onboard sound and had just the green plug plugged into the sound card. Without M3D, I would just get sound out of the front 2 speakers and the sub. With M3D you would get a very very very slight onise out of the rear speakers, but it would be all distorted and scrambled. However, there was a few select songs in which I got actual sound coming out of the rear speakers that wasn't distorted. I then bought a combiner/splitter cable for miniplugs so I could hook up both the green and black inputs from the speakers into the single mini-plug jack on my on board sound. I got the sound out of all 4 speakers without m3d then but with it I still had the popping noise.
I have an audigy now and the same thing happens. When i have both inputs pluggeed into the sound card, everything sounds fine. When I have both inputs and M3D on I still get the scrambled sound and when I have just one input plugged in, and turn m3d I still get the scrambly noise. And one last thing, my "fader" nob seems to be a bit weird. Usually the middle of a fader knob would represent an equal balance between front/back or left/right. However, I only acheive and equal balance when the fader is maxed out.
Try turning your fade knob all the way up. I had a similar problem with mine, but when the fade was turned all the way up, the sound seemed to be distributed evenly through all 4 speakers...D3D was turned on of course.
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I always have my fader knob all the way. And what do you mean by D3D? The only D3D I know is direct 3d