The actors' voices normally come from the center channel of a 5.1 setup. The music generally will use the front stereo pair and subwoofer channel, and often the rear pair as well. If the voices are weak or missing, then your center channel volume is too low or missing. Either because you have it set that way, an audio cable for it is missing, or you don't even have a center channel speaker. Investigate along those lines.
If you don't have a center channel speaker, some 5.1 decoders have a setting to "simulate" or "phantom" the center channel, by taking the center channel audio and feeding it to the two front stereo speakers instead. You can check for that capability if you need it.
Computers are easily mis-configured. If all you have is a pair of stereo speakers on the machine (with or without sub), you have to be sure your computer is NOT set to reproduce 5.1 sound. That will lose the center channel and the rear channel audio. If a movie or whatever with a 5.1 track is played, look in the menu for an audio selection to regular stereo (or Dolby pro-logic, which encodes into two channels).
Hope this helps.