audio problem with dvdS

Using Vista Home/Office. First all my other sound options work ok. I have TV
hooked up thru computer. Sound is fine. Sound is fine for playing audio CD's
and using programs like Rhapsody. Also works OK for playing mpg, vob, etc
files. But when I try to play a DVD movie, the speaking parts are washed
out. Doesn't make a difference whether I use Media Player, Nero, whatever, I
can hear the music and background noises just fine but when then speak it's
barely audible. If I turn up the volume very high, I can hear the speaking,
abet very tiny, but when the background music comes on it blast you out of
the house. Any ideas what I need to go to correct??? TIA.