Partial Audio with MKV files

I did a clean install of windows 6 64bit on a core2duo system. I have installed the Klite codec pack. When I try to play a video file (mkv file) H.264, I get all of the back ground audio but no voice track playing.... ie, sound effects and music but no dialogue. Has anyone else run into this issue? Thanks for any help in solving this problem.

Archer

Im not sure if this will help but I tend to use the shark007 codecs for windows 7 rather than the klite codec pack. There are both x86 & x64 versions and you can have both installed at once. May be worth giving them a try just to see if it kicks in.

x86 (32 bit) Codecs - http://www.majorgeeks.com/Win7codecs_d5959.html
x64 (64 bit) Codecs - http://www.majorgeeks.com/download.php?det=5535

Archer: It sounds like the speaker configuration in the audio codec is set to 5.1, which would send speech to a nonexistent center channel. Poke through the options and set the correct channel count.

I have my computer connected to a surround sound stereo system. The center channel works with other video formats such as AVI. It only seems to be with MKV files. Would changing this option mess up the surround sound processing?