You didn't give us any specific information about your computer's make and model, but you should be able to visit the computer manufacturer's website, then download and install the latest audio driver they have at their site.. Visiting the sound card manufacturer's website might get you the very newest driver for your audio card.. Check the Control Panel/Device Manager to find your audio card manufacturer and the date and driver version number for your card.
The problems you're experiencing could be a failing audio card but it's always good to check to see if an updated driver fixes the issue because it's free..
Hope this helps.
Grif
Few days ago something happened with sound on my PC. I noticed it while playing GTA IV - the dialogues were almost unable to hear while other sound effects were much louder than usual. It affects also on music and movies, and I didn't change anything in any audio options. When I'm listening to mp3 which is really low on quality, I also hear cracks in the background to the distorted music. Please, help!

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