Do you have just 2 speakers? Or do you have surround speakers?
center channel off is a very good idea
In that case I would try checking all the DVD player software settings, or delete and reinstalling the DVD player software. I would also check all the audio software settings, see if the audio is set to surround, or 2 speakers.
Double click the speaker icon in the lower right hand of the desktop screen. The sound software should come up in a window. Carefully check all of the settings. If the system is set to surround, and there are only two speakers, that would explain what is happening. If you have two speakers, turn off the surround settings.
Now test, does a CD played in the DVD drive have the same problems? Or does it sound correct?
If the CD has the same sound problem, and your audio is set for two speakers, surround turned off, then you might actually have a bad DVD drive. It's unusual, but possible.