no speech in some movies

i was watching a few movies today and the last one was a TeleSync with a Mp3 audio source - it played fine.
and after that i wanted to watch a 720p with DTS audio source, but this one had no speech played - thought the release had an error - but after trying other files with AC3(6 channels) i got the same problem - no speech at all. so i figured out it has to do something with the AC3 filter (i guess)
its like only side channels are played on Stereo system. (got only left and right here).

ive tried uninstalling the mega pack and reinstalling the problem is still there.
googled a bit around w/o a result so far.

hope you can help me  

never had this problem with the pack - since im using K-lite mega codec pack since v1.0!

You need to select "Stereo (2.0)" in the codec pack installer (or in the Codec Tweak Tool after install).
You also need to configure your speakers properly in the Windows Control panel. I suspect it is set to 5.1 there.

Once the speakers are configured properly, you should be able to hear the voices. If they are not loud enough, you can use the Volume Normalization option to boost the volume.