As a matter of interest, does this issue apply to specific content such as audio files (MP3, WMA, Wav, etc), the audio soundtrack of video content or both?
If the audio is not working properly regardless of the content such as even including the sound you get when you power down your PC, then either the sound card or speakers are at fault. If your sound card features a graphic equaliser or bass/treble controls (such as a Creative model), adjust these to the middle and switch off any effects if present. Also, try setting the wave and master sound volume levels to middle (if they are turned up high) and turn up the volume on your speakers to compensate. If these suggestions do not help, try plugging one of your speaker cables from the back of your PC into a portable CD player or radio to check if the speakers are at fault.
If MP3 files are at fault, it may be worth trying another software player to play them back. Most playback software including Winamp uses its own codecs, which means that installing Windows codecâ€™s will have no effect if there is a problem. If an MP3 in Winamp comes out bad, then the problem is almost certainly to do with your soundcard driver, soundcard or attached speakers.
As you appear to have uninstalled your video codecâ€™s, this may explain why certain video will no longer play (DivX and AVI files for example). If you reinstall ffdshow with the DivX add-ons, video should playback fine again. Otherwise, try reinstalling your DivX codec. I'm not sure why the ACE Media Pack does not get back your video codec's since this is suppose to include many of the popular AV codecs including DivX. If DVDs no longer play, you will need to reinstall PowerDVD.