Audio Problems: Playing mpg's, etc in various Browsers

I'm experiencing problems with downloading mpg's, wmv's, etc from websites - the video is showing but there is no audio. When I download an mpg - Quicktime just plays the video w/out audio. When I download a wmv, Windows Media Player 9.0 says I do not have the correct codec. I've experienced these problems in Safari and Firefox. BTW, I'm viewing these files from within the browser. I've even noticed that when I visit ESPN.com - the small movie in the upper right hand corner (Macromedia Flash 7) plays video, but no audio.

I used to not have these problems - not sure how I could have "lost" a codec - or if so - how I go about getting it back. I'm not sure if it has something to do with it - but I did recently install and use MacTheRipper to rip some DVD's and transfer them to my PSP. . .

Any help would be appreciated. I've tried google'ing "Quicktime", "Codec", "no audio" - but my research has been fruitless. Thanks so much.