its the fact that people are now encoding with AC3 sound ... and when you play back your only playing it in stereo .. so certain levels are cancelling each other out .. and some channels are quite because its spose to be playing 4 chans at once to make one sound...
Codec packs I find usually fuck things up all together .. how ever I found a good one that I use klmcodec100.exe ..
try this ... its about 16 meg .. but DONT install there crap media player .. just whore up the codecs

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http://www.k-litecodecpack.com/