from the first look i think its all about codecs.
download the latest ones:
http://www.free-codecs.com/download_soft.php?d=4898&s=95
those should help. But also I think u need good player. U always might want to use the one included with codecs and that what u should do for now.
If that work than u might in a future download "Sub-Edit Player" which is my personal favourite, but first i want u to try the codecs.
However do u have a sound at all ?? Did u change ur Operating System in since the problem started ?? those might be important.