I had the same problem. The thing that cured mine was to download 'ACE Mega CoDecS Pack Dashboard', found here:

http://www.freewaresoft.net/

I personally gave it a high rating and recommend it to everyone. BTW it's FREEWARE

It not only gave me the missing Codex, but it lets me choose which player I want to use for playback. Not all music/videos sound the same as others when downloaded from different sources. At least to me anyway.

There are lots of different playbacks you can get and I like to try different ones. I currently use the following:

BSPlayer/Media Player/DivX/Codecs Media Player/Crystal Player/Direct X 9/FastStone Player to name just a few. Of those, the only ones to give less than perfect playback seems to be the FastStone player and the Crystal. All of these came from either www.cnet.com or www.zdnet.com and tuocows.com.

If it helps, great! If it doesn't, well I tried.

Marc001