Could be a few things. Generally speaking, I dislike lots of codecs on systems & loathe codec packs. I would strongly recommend getting rid of codec packs if you have any installed.
The very best audio decoder I have seen & have installed on three machines is FFD tryouts - get it from sourceforge.net, but check the release date as it's been around for a few years. You want at least this year on the release.
The other obvious place to look is your soundcard and drivers. If you're using a creative card and vista, then you will have problems, since creative are behind on fully functional drivers. You don't say what system you've got, but there's a couple of things to look at. Good luck, faulty audio is very frustrating.