This is almost a daily hair-puller for me.
In a surround sound environment, the majority of dialog is mapped to the center channel.
So if 3.1, explosions and rustling leaves from left and right channels, vocals and dialog from the center, and a subwoofer, basically
It's possible that the levels need adjusted to suit the stereo output of your computer.
If your player uses ffdshow, for example, there are adjustments for the different channels.
Which is great if you're watching a sporting event and you want to hear more of the commentators or more of the crowd noise.
The problem is a lot of video that has already been converted ends up only in stereo.
A player I like on the laptop is Media Player Classic- Home Cinema:http://mpc-hc.sourceforge.net/
It'll play darn near everything. And it's one of those things that anyone can use, but pop the hood and there's a ton of customization available.