K-Lite Codec pack will fix any problems with sound.
1. Possible Cause: Missing Codec
Download and install the K-Lite Codec pack. Again, google it. A CODEC stands for COmpressor DECompressor and is a way to make audio and video files small enough to be transferred on the internet. There are a lot of different codecs and if you want to play back a video that was compressed using a particular codec, you will need that same corresponding codec to decompress your media source and view its content. Basically if you dont have a codec, you cant observe the content of the media. If you have a VCR and want to play back a DVD, you are out of luck unless you get a DVD Player. DivX and XVid Codecs are some of the very popular codecs used on the net, but to get those, I still suggest getting the K-Lite Codec Pack.
2. After that download and run Ccleaner and have it remove flash content that is saved on your system. That will fix the youtube problem.
Source:
http://www.adobe.com/cfusion/webforu...0bdaf781772561