I used to be able to play all videos online but a few months back trouble began and now I can't. I have Windows XP. I have tried most every solution I've found online to no avail. The really weird thing is my speakers make an awful sound as well. Not when I play my own music or videos though. But even then once in a while the speakers make a short funny sound. Reminds me of the dial-up noise I used to hear on AOL many years ago. Oh, I've also downloaded and tried just about every codec pack out there...nope!
The noise might be cell phone interference with speakers, it happens.
Usually online videos use plugins such as Shockwave and Flash, update yours for Your browser from Adobe. Check security of your browser, that you did not force online videos to use Windows Media Player exclusively.