Try CodecSniper, at the first link in my previous post. See if you have any "broken" codecs. Careful which ones you delete!
The trouble with codec packs is, the possibility of filter merit issues is enormously increased. When more than one codec can play a file, they may conflict to the point that the file can no longer be rendered. "Broken" codecs are evidence of conflicts, and I'll bet you have some, since you installed a codec pack, unless it's a minimal pack. Again, the conflicts may or may not get to the point that a file cannot be rendered, so most folks don't think there's anything to worry about in regard to codec packs.
Anyway, yes, there may be other factors involved, maybe some one else has some ideas?
Have you tried re-installing your audio? Onboard audio or card? And if nothing else works, try installing AC3Filter. It will assume top priority in the filter chain and perhaps resolve a conflict. Leave the settings at default. Free:
http://ac3filter.net/projects/ac3filter
Good luck.
