XVID Codac on IrfanView (no sound)

Ok, this is actually one of my problems (caused by lazyness of googling) My comp isn't so great when it comes to playing movies, but IrfanView works amazingly with those. Plays any movie. Problem is, I have downloaded some to test it out (south park episodes etc. ) and ones that are like something.xvid.something.avi don't have any sound. Quick time and windows media can't handle it because of file size but Irfan plays it great without any audio. I downloaded Xvid and DivX and neither did anything (DivX plays it with the occasional glitch and after five minutes it skips and crashes while Irfan will play the entire thing no glitches or anything) So I was wondering if anyone knows how to get the codacs working for Irfan or if there is another way to get sound (and no it is not the volume. A non-xvid mpg worked just fine.) Thanks guys/gals for future help (hopefully)