help logitech Z680's and games sound is off

I have Z680's hooked up to my xbox through a digital cable, and while 
playing games no voice comes from the center speaker, only sound 
effects.
Voices are really quite, and if anything is happening at the same time 
someone is talking you can barely hear what they say, so I have to turn 
it up really loud, and then explosions and things like that are even 
louder.
This happens on all my games and DVDs, can anyone help me figure out why 
this is happening?

If I plug my xbox into my TV and just use my TV speakers voice levels 
are fine.
Same thing with gamecube I bought a Y adapter and hooked it up and some 
thign swere loud and others were quiet.
Thanks for the replies in advance.