CMSS is a feature which 'up-mixes' stereo sources so that they seem like 5.1 audio. 'up-mixing' is a feature found on most decent home entertainment systems. im not quite an expert, but i believe it shifts sounds with certain frequency ranges to the unused speakers(back and center). so, in other words, with stereo sounds like music and movies that only go to 2 channels, turn it on for a little more immersive sound. with games and dvds and such its worthless, since they already go to all channels. if you dont really care and/or your speaker setup sucks, just leave it off.
and your other question, AC3 is another word for dolby surround or multi-channel audio. so yes, its perferable to just plain ol stereo, as long as you got a 5.1 setup.
Core i7 920 @ 3.8Ghz | 6GB OCZ DDR3 8-8-8-24 @ 1600mhz | eVga x58 Mobo | 2 x eVga GTX 460 SLI | Intel X25-M + 3x Seagate + WD Black = 2.75TB | X-Fi Titanium | PCP&C Silencer 750 | G15 KB | G5 Mouse | G35 Headset | Z-5500 Digital | Samsung T260HD
). But I just got an Audigy-2-ZS Platinum retail on the cheap and was a little worried as to whether what I'm hearing is what I should be hearing is all.
