1) Then you could have your speakers wired incorrectly, to the wrong outputs, or since MPC-HC works, you probably have some channel re-mapping in MC's DSP. Check your DSP, or share details of what is turned on, and what is set in there.
2) Are those problem movies stereo only, or multi-channel? Stereo movies, or TV, depending on the source, can be very poorly down mixed from multi-channel sources. Does MPC-HC really handle these movies better, with clear speech? Again, if they are multi-channel you have some strange channel mapping in MC's DSP, anything could be happening to the Centre channel, which usually carries the dialogue.
3) What is in your DSP? What does the Audio Path show is being done to the source audio when playing a problem file? Do you have some reverb turned on somewhere? Have you selected a Concert Hall sound effect or similar? What are you outputting to in MC, and does it use Direct Sound, and if so, do you have any sound effects added in the Windows Playback device?
As Brian said, more details would help.