Sound issues when playing movies - can hear background music but not dialogue

Hello, I just wiped my computer due to poor performance and have gotten all the drivers reloaded via the Dell website (I have a Dell Inspiron E1705 with XP and Media Center 2005).  When I go to play DVDs, I can hear the music at the appropriate level but the dialogue is so quiet that you have to turn the volume all the way up and its still quiet (but the background music is BLASTING).  Does anyone have any suggestions for fixing this?  I have re-downloaded drivers and have looked everywhere for an answer - please help!  Thanks!