I would appreciate help with sound problem

I have downloaded lost episode:
Lost.S03E01.A.Tale.of.Two.Cities.HR.HDTV.AC3.5.1.XviD-CTU.avi

I can see the picture, and hear the music, but I cant hear any of the speech.
It is so frustrating..

Please help me

thanks

i have a problem with sound on my computer when playing movies